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  1.  
  2. Spoorstraat, Geleen
  3.  
  4.  
  5.  
  6. https://tools.ietf.org/html/rfc821
  7.  
  8.  
  9.  
  10.  
  11. [Docs] [txt|pdf] [Tracker]
  12.  
  13. Obsoleted by: 2821 INTERNET STANDARD
  14.  
  15.  
  16. RFC 821
  17.  
  18.  
  19.  
  20.  
  21.  
  22. SIMPLE MAIL TRANSFER PROTOCOL
  23.  
  24.  
  25.  
  26. Jonathan B. Postel
  27.  
  28.  
  29.  
  30.  
  31.  
  32.  
  33.  
  34.  
  35.  
  36.  
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  45.  
  46.  
  47.  
  48.  
  49.  
  50.  
  51.  
  52.  
  53.  
  54.  
  55.  
  56. August 1982
  57.  
  58.  
  59.  
  60. Information Sciences Institute
  61. University of Southern California
  62. 4676 Admiralty Way
  63. Marina del Rey, California 90291
  64.  
  65. (213) 822-1511
  66.  
  67.  
  68. RFC 821 August 1982
  69. Simple Mail Transfer Protocol
  70.  
  71.  
  72.  
  73. TABLE OF CONTENTS
  74.  
  75. 1. INTRODUCTION .................................................. 1
  76.  
  77. 2. THE SMTP MODEL ................................................ 2
  78.  
  79. 3. THE SMTP PROCEDURE ............................................ 4
  80.  
  81. 3.1. Mail ..................................................... 4
  82. 3.2. Forwarding ............................................... 7
  83. 3.3. Verifying and Expanding .................................. 8
  84. 3.4. Sending and Mailing ..................................... 11
  85. 3.5. Opening and Closing ..................................... 13
  86. 3.6. Relaying ................................................ 14
  87. 3.7. Domains ................................................. 17
  88. 3.8. Changing Roles .......................................... 18
  89.  
  90. 4. THE SMTP SPECIFICATIONS ...................................... 19
  91.  
  92. 4.1. SMTP Commands ........................................... 19
  93. 4.1.1. Command Semantics ..................................... 19
  94. 4.1.2. Command Syntax ........................................ 27
  95. 4.2. SMTP Replies ............................................ 34
  96. 4.2.1. Reply Codes by Function Group ......................... 35
  97. 4.2.2. Reply Codes in Numeric Order .......................... 36
  98. 4.3. Sequencing of Commands and Replies ...................... 37
  99. 4.4. State Diagrams .......................................... 39
  100. 4.5. Details ................................................. 41
  101. 4.5.1. Minimum Implementation ................................ 41
  102. 4.5.2. Transparency .......................................... 41
  103. 4.5.3. Sizes ................................................. 42
  104.  
  105. APPENDIX A: TCP ................................................. 44
  106. APPENDIX B: NCP ................................................. 45
  107. APPENDIX C: NITS ................................................ 46
  108. APPENDIX D: X.25 ................................................ 47
  109. APPENDIX E: Theory of Reply Codes ............................... 48
  110. APPENDIX F: Scenarios ........................................... 51
  111.  
  112. GLOSSARY ......................................................... 64
  113.  
  114. REFERENCES ....................................................... 67
  115.  
  116.  
  117.  
  118.  
  119. Network Working Group J. Postel
  120. Request for Comments: DRAFT ISI
  121. Replaces: RFC 788, 780, 772 August 1982
  122.  
  123. SIMPLE MAIL TRANSFER PROTOCOL
  124.  
  125.  
  126. 1. INTRODUCTION
  127.  
  128. The objective of Simple Mail Transfer Protocol (SMTP) is to transfer
  129. mail reliably and efficiently.
  130.  
  131. SMTP is independent of the particular transmission subsystem and
  132. requires only a reliable ordered data stream channel. Appendices A,
  133. B, C, and D describe the use of SMTP with various transport services.
  134. A Glossary provides the definitions of terms as used in this
  135. document.
  136.  
  137. An important feature of SMTP is its capability to relay mail across
  138. transport service environments. A transport service provides an
  139. interprocess communication environment (IPCE). An IPCE may cover one
  140. network, several networks, or a subset of a network. It is important
  141. to realize that transport systems (or IPCEs) are not one-to-one with
  142. networks. A process can communicate directly with another process
  143. through any mutually known IPCE. Mail is an application or use of
  144. interprocess communication. Mail can be communicated between
  145. processes in different IPCEs by relaying through a process connected
  146. to two (or more) IPCEs. More specifically, mail can be relayed
  147. between hosts on different transport systems by a host on both
  148. transport systems.
  149.  
  150.  
  151.  
  152.  
  153.  
  154.  
  155.  
  156.  
  157.  
  158.  
  159.  
  160.  
  161.  
  162.  
  163.  
  164.  
  165.  
  166.  
  167.  
  168.  
  169.  
  170.  
  171.  
  172.  
  173. Postel [Page 1]
  174.  
  175.  
  176. August 1982 RFC 821
  177. Simple Mail Transfer Protocol
  178.  
  179.  
  180.  
  181. 2. THE SMTP MODEL
  182.  
  183. The SMTP design is based on the following model of communication: as
  184. the result of a user mail request, the sender-SMTP establishes a
  185. two-way transmission channel to a receiver-SMTP. The receiver-SMTP
  186. may be either the ultimate destination or an intermediate. SMTP
  187. commands are generated by the sender-SMTP and sent to the
  188. receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
  189. sender-SMTP in response to the commands.
  190.  
  191. Once the transmission channel is established, the SMTP-sender sends a
  192. MAIL command indicating the sender of the mail. If the SMTP-receiver
  193. can accept mail it responds with an OK reply. The SMTP-sender then
  194. sends a RCPT command identifying a recipient of the mail. If the
  195. SMTP-receiver can accept mail for that recipient it responds with an
  196. OK reply; if not, it responds with a reply rejecting that recipient
  197. (but not the whole mail transaction). The SMTP-sender and
  198. SMTP-receiver may negotiate several recipients. When the recipients
  199. have been negotiated the SMTP-sender sends the mail data, terminating
  200. with a special sequence. If the SMTP-receiver successfully processes
  201. the mail data it responds with an OK reply. The dialog is purposely
  202. lock-step, one-at-a-time.
  203.  
  204. -------------------------------------------------------------
  205.  
  206.  
  207. +----------+ +----------+
  208. +------+ | | | |
  209. | User |<-->| | SMTP | |
  210. +------+ | Sender- |Commands/Replies| Receiver-|
  211. +------+ | SMTP |<-------------->| SMTP | +------+
  212. | File |<-->| | and Mail | |<-->| File |
  213. |System| | | | | |System|
  214. +------+ +----------+ +----------+ +------+
  215.  
  216.  
  217. Sender-SMTP Receiver-SMTP
  218.  
  219. Model for SMTP Use
  220.  
  221. Figure 1
  222.  
  223. -------------------------------------------------------------
  224.  
  225. The SMTP provides mechanisms for the transmission of mail; directly
  226. from the sending user's host to the receiving user's host when the
  227.  
  228.  
  229.  
  230. [Page 2] Postel
  231.  
  232.  
  233.  
  234.  
  235. RFC 821 August 1982
  236. Simple Mail Transfer Protocol
  237.  
  238.  
  239.  
  240. two host are connected to the same transport service, or via one or
  241. more relay SMTP-servers when the source and destination hosts are not
  242. connected to the same transport service.
  243.  
  244. To be able to provide the relay capability the SMTP-server must be
  245. supplied with the name of the ultimate destination host as well as
  246. the destination mailbox name.
  247.  
  248. The argument to the MAIL command is a reverse-path, which specifies
  249. who the mail is from. The argument to the RCPT command is a
  250. forward-path, which specifies who the mail is to. The forward-path
  251. is a source route, while the reverse-path is a return route (which
  252. may be used to return a message to the sender when an error occurs
  253. with a relayed message).
  254.  
  255. When the same message is sent to multiple recipients the SMTP
  256. encourages the transmission of only one copy of the data for all the
  257. recipients at the same destination host.
  258.  
  259. The mail commands and replies have a rigid syntax. Replies also have
  260. a numeric code. In the following, examples appear which use actual
  261. commands and replies. The complete lists of commands and replies
  262. appears in Section 4 on specifications.
  263.  
  264. Commands and replies are not case sensitive. That is, a command or
  265. reply word may be upper case, lower case, or any mixture of upper and
  266. lower case. Note that this is not true of mailbox user names. For
  267. some hosts the user name is case sensitive, and SMTP implementations
  268. must take case to preserve the case of user names as they appear in
  269. mailbox arguments. Host names are not case sensitive.
  270.  
  271. Commands and replies are composed of characters from the ASCII
  272. character set [1]. When the transport service provides an 8-bit byte
  273. (octet) transmission channel, each 7-bit character is transmitted
  274. right justified in an octet with the high order bit cleared to zero.
  275.  
  276. When specifying the general form of a command or reply, an argument
  277. (or special symbol) will be denoted by a meta-linguistic variable (or
  278. constant), for example, "<string>" or "<reverse-path>". Here the
  279. angle brackets indicate these are meta-linguistic variables.
  280. However, some arguments use the angle brackets literally. For
  281. example, an actual reverse-path is enclosed in angle brackets, i.e.,
  282. "<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
  283. angle brackets are actually transmitted in the command or reply).
  284.  
  285.  
  286.  
  287.  
  288.  
  289. Postel [Page 3]
  290.  
  291.  
  292. August 1982 RFC 821
  293. Simple Mail Transfer Protocol
  294.  
  295.  
  296.  
  297. 3. THE SMTP PROCEDURES
  298.  
  299. This section presents the procedures used in SMTP in several parts.
  300. First comes the basic mail procedure defined as a mail transaction.
  301. Following this are descriptions of forwarding mail, verifying mailbox
  302. names and expanding mailing lists, sending to terminals instead of or
  303. in combination with mailboxes, and the opening and closing exchanges.
  304. At the end of this section are comments on relaying, a note on mail
  305. domains, and a discussion of changing roles. Throughout this section
  306. are examples of partial command and reply sequences, several complete
  307. scenarios are presented in Appendix F.
  308.  
  309. 3.1. MAIL
  310.  
  311. There are three steps to SMTP mail transactions. The transaction
  312. is started with a MAIL command which gives the sender
  313. identification. A series of one or more RCPT commands follows
  314. giving the receiver information. Then a DATA command gives the
  315. mail data. And finally, the end of mail data indicator confirms
  316. the transaction.
  317.  
  318. The first step in the procedure is the MAIL command. The
  319. <reverse-path> contains the source mailbox.
  320.  
  321. MAIL <SP> FROM:<reverse-path> <CRLF>
  322.  
  323. This command tells the SMTP-receiver that a new mail
  324. transaction is starting and to reset all its state tables and
  325. buffers, including any recipients or mail data. It gives the
  326. reverse-path which can be used to report errors. If accepted,
  327. the receiver-SMTP returns a 250 OK reply.
  328.  
  329. The <reverse-path> can contain more than just a mailbox. The
  330. <reverse-path> is a reverse source routing list of hosts and
  331. source mailbox. The first host in the <reverse-path> should be
  332. the host sending this command.
  333.  
  334. The second step in the procedure is the RCPT command.
  335.  
  336. RCPT <SP> TO:<forward-path> <CRLF>
  337.  
  338. This command gives a forward-path identifying one recipient.
  339. If accepted, the receiver-SMTP returns a 250 OK reply, and
  340. stores the forward-path. If the recipient is unknown the
  341. receiver-SMTP returns a 550 Failure reply. This second step of
  342. the procedure can be repeated any number of times.
  343.  
  344.  
  345.  
  346. [Page 4] Postel
  347.  
  348.  
  349.  
  350.  
  351. RFC 821 August 1982
  352. Simple Mail Transfer Protocol
  353.  
  354.  
  355.  
  356. The <forward-path> can contain more than just a mailbox. The
  357. <forward-path> is a source routing list of hosts and the
  358. destination mailbox. The first host in the <forward-path>
  359. should be the host receiving this command.
  360.  
  361. The third step in the procedure is the DATA command.
  362.  
  363. DATA <CRLF>
  364.  
  365. If accepted, the receiver-SMTP returns a 354 Intermediate reply
  366. and considers all succeeding lines to be the message text.
  367. When the end of text is received and stored the SMTP-receiver
  368. sends a 250 OK reply.
  369.  
  370. Since the mail data is sent on the transmission channel the end
  371. of the mail data must be indicated so that the command and
  372. reply dialog can be resumed. SMTP indicates the end of the
  373. mail data by sending a line containing only a period. A
  374. transparency procedure is used to prevent this from interfering
  375. with the user's text (see Section 4.5.2).
  376.  
  377. Please note that the mail data includes the memo header
  378. items such as Date, Subject, To, Cc, From [2].
  379.  
  380. The end of mail data indicator also confirms the mail
  381. transaction and tells the receiver-SMTP to now process the
  382. stored recipients and mail data. If accepted, the
  383. receiver-SMTP returns a 250 OK reply. The DATA command should
  384. fail only if the mail transaction was incomplete (for example,
  385. no recipients), or if resources are not available.
  386.  
  387. The above procedure is an example of a mail transaction. These
  388. commands must be used only in the order discussed above.
  389. Example 1 (below) illustrates the use of these commands in a mail
  390. transaction.
  391.  
  392.  
  393.  
  394.  
  395.  
  396.  
  397.  
  398.  
  399.  
  400.  
  401.  
  402.  
  403.  
  404.  
  405. Postel [Page 5]
  406.  
  407.  
  408. August 1982 RFC 821
  409. Simple Mail Transfer Protocol
  410.  
  411.  
  412.  
  413. -------------------------------------------------------------
  414.  
  415. Example of the SMTP Procedure
  416.  
  417. This SMTP example shows mail sent by Smith at host Alpha.ARPA,
  418. to Jones, Green, and Brown at host Beta.ARPA. Here we assume
  419. that host Alpha contacts host Beta directly.
  420.  
  421. S: MAIL FROM:<Smith@Alpha.ARPA>
  422. R: 250 OK
  423.  
  424. S: RCPT TO:<Jones@Beta.ARPA>
  425. R: 250 OK
  426.  
  427. S: RCPT TO:<Green@Beta.ARPA>
  428. R: 550 No such user here
  429.  
  430. S: RCPT TO:<Brown@Beta.ARPA>
  431. R: 250 OK
  432.  
  433. S: DATA
  434. R: 354 Start mail input; end with <CRLF>.<CRLF>
  435. S: Blah blah blah...
  436. S: ...etc. etc. etc.
  437. S: <CRLF>.<CRLF>
  438. R: 250 OK
  439.  
  440. The mail has now been accepted for Jones and Brown. Green did
  441. not have a mailbox at host Beta.
  442.  
  443. Example 1
  444.  
  445. -------------------------------------------------------------
  446.  
  447.  
  448.  
  449.  
  450.  
  451.  
  452.  
  453.  
  454.  
  455.  
  456.  
  457.  
  458.  
  459.  
  460.  
  461.  
  462. [Page 6] Postel
  463.  
  464.  
  465.  
  466.  
  467. RFC 821 August 1982
  468. Simple Mail Transfer Protocol
  469.  
  470.  
  471.  
  472. 3.2. FORWARDING
  473.  
  474. There are some cases where the destination information in the
  475. <forward-path> is incorrect, but the receiver-SMTP knows the
  476. correct destination. In such cases, one of the following replies
  477. should be used to allow the sender to contact the correct
  478. destination.
  479.  
  480. 251 User not local; will forward to <forward-path>
  481.  
  482. This reply indicates that the receiver-SMTP knows the user's
  483. mailbox is on another host and indicates the correct
  484. forward-path to use in the future. Note that either the
  485. host or user or both may be different. The receiver takes
  486. responsibility for delivering the message.
  487.  
  488. 551 User not local; please try <forward-path>
  489.  
  490. This reply indicates that the receiver-SMTP knows the user's
  491. mailbox is on another host and indicates the correct
  492. forward-path to use. Note that either the host or user or
  493. both may be different. The receiver refuses to accept mail
  494. for this user, and the sender must either redirect the mail
  495. according to the information provided or return an error
  496. response to the originating user.
  497.  
  498. Example 2 illustrates the use of these responses.
  499.  
  500. -------------------------------------------------------------
  501.  
  502. Example of Forwarding
  503.  
  504. Either
  505.  
  506. S: RCPT TO:<Postel@USC-ISI.ARPA>
  507. R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>
  508.  
  509. Or
  510.  
  511. S: RCPT TO:<Paul@USC-ISIB.ARPA>
  512. R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>
  513.  
  514. Example 2
  515.  
  516. -------------------------------------------------------------
  517.  
  518.  
  519.  
  520.  
  521. Postel [Page 7]
  522.  
  523.  
  524. August 1982 RFC 821
  525. Simple Mail Transfer Protocol
  526.  
  527.  
  528.  
  529. 3.3. VERIFYING AND EXPANDING
  530.  
  531. SMTP provides as additional features, commands to verify a user
  532. name or expand a mailing list. This is done with the VRFY and
  533. EXPN commands, which have character string arguments. For the
  534. VRFY command, the string is a user name, and the response may
  535. include the full name of the user and must include the mailbox of
  536. the user. For the EXPN command, the string identifies a mailing
  537. list, and the multiline response may include the full name of the
  538. users and must give the mailboxes on the mailing list.
  539.  
  540. "User name" is a fuzzy term and used purposely. If a host
  541. implements the VRFY or EXPN commands then at least local mailboxes
  542. must be recognized as "user names". If a host chooses to
  543. recognize other strings as "user names" that is allowed.
  544.  
  545. In some hosts the distinction between a mailing list and an alias
  546. for a single mailbox is a bit fuzzy, since a common data structure
  547. may hold both types of entries, and it is possible to have mailing
  548. lists of one mailbox. If a request is made to verify a mailing
  549. list a positive response can be given if on receipt of a message
  550. so addressed it will be delivered to everyone on the list,
  551. otherwise an error should be reported (e.g., "550 That is a
  552. mailing list, not a user"). If a request is made to expand a user
  553. name a positive response can be formed by returning a list
  554. containing one name, or an error can be reported (e.g., "550 That
  555. is a user name, not a mailing list").
  556.  
  557. In the case of a multiline reply (normal for EXPN) exactly one
  558. mailbox is to be specified on each line of the reply. In the case
  559. of an ambiguous request, for example, "VRFY Smith", where there
  560. are two Smith's the response must be "553 User ambiguous".
  561.  
  562. The case of verifying a user name is straightforward as shown in
  563. example 3.
  564.  
  565.  
  566.  
  567.  
  568.  
  569.  
  570.  
  571.  
  572.  
  573.  
  574.  
  575.  
  576.  
  577.  
  578. [Page 8] Postel
  579.  
  580.  
  581.  
  582.  
  583. RFC 821 August 1982
  584. Simple Mail Transfer Protocol
  585.  
  586.  
  587.  
  588. -------------------------------------------------------------
  589.  
  590. Example of Verifying a User Name
  591.  
  592. Either
  593.  
  594. S: VRFY Smith
  595. R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
  596.  
  597. Or
  598.  
  599. S: VRFY Smith
  600. R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
  601.  
  602. Or
  603.  
  604. S: VRFY Jones
  605. R: 550 String does not match anything.
  606.  
  607. Or
  608.  
  609. S: VRFY Jones
  610. R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
  611.  
  612. Or
  613.  
  614. S: VRFY Gourzenkyinplatz
  615. R: 553 User ambiguous.
  616.  
  617. Example 3
  618.  
  619. -------------------------------------------------------------
  620.  
  621.  
  622.  
  623.  
  624.  
  625.  
  626.  
  627.  
  628.  
  629.  
  630.  
  631.  
  632.  
  633.  
  634.  
  635.  
  636.  
  637. Postel [Page 9]
  638.  
  639.  
  640. August 1982 RFC 821
  641. Simple Mail Transfer Protocol
  642.  
  643.  
  644.  
  645. The case of expanding a mailbox list requires a multiline reply as
  646. shown in example 4.
  647.  
  648. -------------------------------------------------------------
  649.  
  650. Example of Expanding a Mailing List
  651.  
  652. Either
  653.  
  654. S: EXPN Example-People
  655. R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
  656. R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
  657. R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
  658. R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
  659. R: 250-<joe@foo-unix.ARPA>
  660. R: 250 <xyz@bar-unix.ARPA>
  661.  
  662. Or
  663.  
  664. S: EXPN Executive-Washroom-List
  665. R: 550 Access Denied to You.
  666.  
  667. Example 4
  668.  
  669. -------------------------------------------------------------
  670.  
  671. The character string arguments of the VRFY and EXPN commands
  672. cannot be further restricted due to the variety of implementations
  673. of the user name and mailbox list concepts. On some systems it
  674. may be appropriate for the argument of the EXPN command to be a
  675. file name for a file containing a mailing list, but again there is
  676. a variety of file naming conventions in the Internet.
  677.  
  678. The VRFY and EXPN commands are not included in the minimum
  679. implementation (Section 4.5.1), and are not required to work
  680. across relays when they are implemented.
  681.  
  682.  
  683.  
  684.  
  685.  
  686.  
  687.  
  688.  
  689.  
  690.  
  691.  
  692.  
  693.  
  694. [Page 10] Postel
  695.  
  696.  
  697.  
  698.  
  699. RFC 821 August 1982
  700. Simple Mail Transfer Protocol
  701.  
  702.  
  703.  
  704. 3.4. SENDING AND MAILING
  705.  
  706. The main purpose of SMTP is to deliver messages to user's
  707. mailboxes. A very similar service provided by some hosts is to
  708. deliver messages to user's terminals (provided the user is active
  709. on the host). The delivery to the user's mailbox is called
  710. "mailing", the delivery to the user's terminal is called
  711. "sending". Because in many hosts the implementation of sending is
  712. nearly identical to the implementation of mailing these two
  713. functions are combined in SMTP. However the sending commands are
  714. not included in the required minimum implementation
  715. (Section 4.5.1). Users should have the ability to control the
  716. writing of messages on their terminals. Most hosts permit the
  717. users to accept or refuse such messages.
  718.  
  719. The following three command are defined to support the sending
  720. options. These are used in the mail transaction instead of the
  721. MAIL command and inform the receiver-SMTP of the special semantics
  722. of this transaction:
  723.  
  724. SEND <SP> FROM:<reverse-path> <CRLF>
  725.  
  726. The SEND command requires that the mail data be delivered to
  727. the user's terminal. If the user is not active (or not
  728. accepting terminal messages) on the host a 450 reply may
  729. returned to a RCPT command. The mail transaction is
  730. successful if the message is delivered the terminal.
  731.  
  732. SOML <SP> FROM:<reverse-path> <CRLF>
  733.  
  734. The Send Or MaiL command requires that the mail data be
  735. delivered to the user's terminal if the user is active (and
  736. accepting terminal messages) on the host. If the user is
  737. not active (or not accepting terminal messages) then the
  738. mail data is entered into the user's mailbox. The mail
  739. transaction is successful if the message is delivered either
  740. to the terminal or the mailbox.
  741.  
  742. SAML <SP> FROM:<reverse-path> <CRLF>
  743.  
  744. The Send And MaiL command requires that the mail data be
  745. delivered to the user's terminal if the user is active (and
  746. accepting terminal messages) on the host. In any case the
  747. mail data is entered into the user's mailbox. The mail
  748. transaction is successful if the message is delivered the
  749. mailbox.
  750.  
  751.  
  752.  
  753. Postel [Page 11]
  754.  
  755.  
  756. August 1982 RFC 821
  757. Simple Mail Transfer Protocol
  758.  
  759.  
  760.  
  761. The same reply codes that are used for the MAIL commands are used
  762. for these commands.
  763.  
  764.  
  765.  
  766.  
  767.  
  768.  
  769.  
  770.  
  771.  
  772.  
  773.  
  774.  
  775.  
  776.  
  777.  
  778.  
  779.  
  780.  
  781.  
  782.  
  783.  
  784.  
  785.  
  786.  
  787.  
  788.  
  789.  
  790.  
  791.  
  792.  
  793.  
  794.  
  795.  
  796.  
  797.  
  798.  
  799.  
  800.  
  801.  
  802.  
  803.  
  804.  
  805.  
  806.  
  807.  
  808.  
  809.  
  810. [Page 12] Postel
  811.  
  812.  
  813.  
  814.  
  815. RFC 821 August 1982
  816. Simple Mail Transfer Protocol
  817.  
  818.  
  819.  
  820. 3.5. OPENING AND CLOSING
  821.  
  822. At the time the transmission channel is opened there is an
  823. exchange to ensure that the hosts are communicating with the hosts
  824. they think they are.
  825.  
  826. The following two commands are used in transmission channel
  827. opening and closing:
  828.  
  829. HELO <SP> <domain> <CRLF>
  830.  
  831. QUIT <CRLF>
  832.  
  833. In the HELO command the host sending the command identifies
  834. itself; the command may be interpreted as saying "Hello, I am
  835. <domain>".
  836.  
  837. -------------------------------------------------------------
  838.  
  839. Example of Connection Opening
  840.  
  841. R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
  842. S: HELO USC-ISIF.ARPA
  843. R: 250 BBN-UNIX.ARPA
  844.  
  845. Example 5
  846.  
  847. -------------------------------------------------------------
  848.  
  849. -------------------------------------------------------------
  850.  
  851. Example of Connection Closing
  852.  
  853. S: QUIT
  854. R: 221 BBN-UNIX.ARPA Service closing transmission channel
  855.  
  856. Example 6
  857.  
  858. -------------------------------------------------------------
  859.  
  860.  
  861.  
  862.  
  863.  
  864.  
  865.  
  866.  
  867.  
  868.  
  869. Postel [Page 13]
  870.  
  871.  
  872. August 1982 RFC 821
  873. Simple Mail Transfer Protocol
  874.  
  875.  
  876.  
  877. 3.6. RELAYING
  878.  
  879. The forward-path may be a source route of the form
  880. "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. This
  881. form is used to emphasize the distinction between an address and a
  882. route. The mailbox is an absolute address, and the route is
  883. information about how to get there. The two concepts should not
  884. be confused.
  885.  
  886. Conceptually the elements of the forward-path are moved to the
  887. reverse-path as the message is relayed from one server-SMTP to
  888. another. The reverse-path is a reverse source route, (i.e., a
  889. source route from the current location of the message to the
  890. originator of the message). When a server-SMTP deletes its
  891. identifier from the forward-path and inserts it into the
  892. reverse-path, it must use the name it is known by in the
  893. environment it is sending into, not the environment the mail came
  894. from, in case the server-SMTP is known by different names in
  895. different environments.
  896.  
  897. If when the message arrives at an SMTP the first element of the
  898. forward-path is not the identifier of that SMTP the element is not
  899. deleted from the forward-path and is used to determine the next
  900. SMTP to send the message to. In any case, the SMTP adds its own
  901. identifier to the reverse-path.
  902.  
  903. Using source routing the receiver-SMTP receives mail to be relayed
  904. to another server-SMTP The receiver-SMTP may accept or reject the
  905. task of relaying the mail in the same way it accepts or rejects
  906. mail for a local user. The receiver-SMTP transforms the command
  907. arguments by moving its own identifier from the forward-path to
  908. the beginning of the reverse-path. The receiver-SMTP then becomes
  909. a sender-SMTP, establishes a transmission channel to the next SMTP
  910. in the forward-path, and sends it the mail.
  911.  
  912. The first host in the reverse-path should be the host sending the
  913. SMTP commands, and the first host in the forward-path should be
  914. the host receiving the SMTP commands.
  915.  
  916. Notice that the forward-path and reverse-path appear in the SMTP
  917. commands and replies, but not necessarily in the message. That
  918. is, there is no need for these paths and especially this syntax to
  919. appear in the "To:" , "From:", "CC:", etc. fields of the message
  920. header.
  921.  
  922. If a server-SMTP has accepted the task of relaying the mail and
  923.  
  924.  
  925.  
  926. [Page 14] Postel
  927.  
  928.  
  929.  
  930.  
  931. RFC 821 August 1982
  932. Simple Mail Transfer Protocol
  933.  
  934.  
  935.  
  936. later finds that the forward-path is incorrect or that the mail
  937. cannot be delivered for whatever reason, then it must construct an
  938. "undeliverable mail" notification message and send it to the
  939. originator of the undeliverable mail (as indicated by the
  940. reverse-path).
  941.  
  942. This notification message must be from the server-SMTP at this
  943. host. Of course, server-SMTPs should not send notification
  944. messages about problems with notification messages. One way to
  945. prevent loops in error reporting is to specify a null reverse-path
  946. in the MAIL command of a notification message. When such a
  947. message is relayed it is permissible to leave the reverse-path
  948. null. A MAIL command with a null reverse-path appears as follows:
  949.  
  950. MAIL FROM:<>
  951.  
  952. An undeliverable mail notification message is shown in example 7.
  953. This notification is in response to a message originated by JOE at
  954. HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
  955. to HOSTZ. What we see in the example is the transaction between
  956. HOSTY and HOSTX, which is the first step in the return of the
  957. notification message.
  958.  
  959.  
  960.  
  961.  
  962.  
  963.  
  964.  
  965.  
  966.  
  967.  
  968.  
  969.  
  970.  
  971.  
  972.  
  973.  
  974.  
  975.  
  976.  
  977.  
  978.  
  979.  
  980.  
  981.  
  982.  
  983.  
  984.  
  985. Postel [Page 15]
  986.  
  987.  
  988. August 1982 RFC 821
  989. Simple Mail Transfer Protocol
  990.  
  991.  
  992.  
  993. -------------------------------------------------------------
  994.  
  995. Example Undeliverable Mail Notification Message
  996.  
  997. S: MAIL FROM:<>
  998. R: 250 ok
  999. S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
  1000. R: 250 ok
  1001. S: DATA
  1002. R: 354 send the mail data, end with .
  1003. S: Date: 23 Oct 81 11:22:33
  1004. S: From: SMTP@HOSTY.ARPA
  1005. S: To: JOE@HOSTW.ARPA
  1006. S: Subject: Mail System Problem
  1007. S:
  1008. S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
  1009. S: HOSTZ.ARPA said this:
  1010. S: "550 No Such User"
  1011. S: .
  1012. R: 250 ok
  1013.  
  1014. Example 7
  1015.  
  1016. -------------------------------------------------------------
  1017.  
  1018.  
  1019.  
  1020.  
  1021.  
  1022.  
  1023.  
  1024.  
  1025.  
  1026.  
  1027.  
  1028.  
  1029.  
  1030.  
  1031.  
  1032.  
  1033.  
  1034.  
  1035.  
  1036.  
  1037.  
  1038.  
  1039.  
  1040.  
  1041.  
  1042. [Page 16] Postel
  1043.  
  1044.  
  1045.  
  1046.  
  1047. RFC 821 August 1982
  1048. Simple Mail Transfer Protocol
  1049.  
  1050.  
  1051.  
  1052. 3.7. DOMAINS
  1053.  
  1054. Domains are a recently introduced concept in the ARPA Internet
  1055. mail system. The use of domains changes the address space from a
  1056. flat global space of simple character string host names to a
  1057. hierarchically structured rooted tree of global addresses. The
  1058. host name is replaced by a domain and host designator which is a
  1059. sequence of domain element strings separated by periods with the
  1060. understanding that the domain elements are ordered from the most
  1061. specific to the most general.
  1062.  
  1063. For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
  1064. "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
  1065.  
  1066. Whenever domain names are used in SMTP only the official names are
  1067. used, the use of nicknames or aliases is not allowed.
  1068.  
  1069.  
  1070.  
  1071.  
  1072.  
  1073.  
  1074.  
  1075.  
  1076.  
  1077.  
  1078.  
  1079.  
  1080.  
  1081.  
  1082.  
  1083.  
  1084.  
  1085.  
  1086.  
  1087.  
  1088.  
  1089.  
  1090.  
  1091.  
  1092.  
  1093.  
  1094.  
  1095.  
  1096.  
  1097.  
  1098.  
  1099.  
  1100.  
  1101. Postel [Page 17]
  1102.  
  1103.  
  1104. August 1982 RFC 821
  1105. Simple Mail Transfer Protocol
  1106.  
  1107.  
  1108.  
  1109. 3.8. CHANGING ROLES
  1110.  
  1111. The TURN command may be used to reverse the roles of the two
  1112. programs communicating over the transmission channel.
  1113.  
  1114. If program-A is currently the sender-SMTP and it sends the TURN
  1115. command and receives an ok reply (250) then program-A becomes the
  1116. receiver-SMTP.
  1117.  
  1118. If program-B is currently the receiver-SMTP and it receives the
  1119. TURN command and sends an ok reply (250) then program-B becomes
  1120. the sender-SMTP.
  1121.  
  1122. To refuse to change roles the receiver sends the 502 reply.
  1123.  
  1124. Please note that this command is optional. It would not normally
  1125. be used in situations where the transmission channel is TCP.
  1126. However, when the cost of establishing the transmission channel is
  1127. high, this command may be quite useful. For example, this command
  1128. may be useful in supporting be mail exchange using the public
  1129. switched telephone system as a transmission channel, especially if
  1130. some hosts poll other hosts for mail exchanges.
  1131.  
  1132.  
  1133.  
  1134.  
  1135.  
  1136.  
  1137.  
  1138.  
  1139.  
  1140.  
  1141.  
  1142.  
  1143.  
  1144.  
  1145.  
  1146.  
  1147.  
  1148.  
  1149.  
  1150.  
  1151.  
  1152.  
  1153.  
  1154.  
  1155.  
  1156.  
  1157.  
  1158. [Page 18] Postel
  1159.  
  1160.  
  1161.  
  1162.  
  1163. RFC 821 August 1982
  1164. Simple Mail Transfer Protocol
  1165.  
  1166.  
  1167.  
  1168. 4. THE SMTP SPECIFICATIONS
  1169.  
  1170. 4.1. SMTP COMMANDS
  1171.  
  1172. 4.1.1. COMMAND SEMANTICS
  1173.  
  1174. The SMTP commands define the mail transfer or the mail system
  1175. function requested by the user. SMTP commands are character
  1176. strings terminated by <CRLF>. The command codes themselves are
  1177. alphabetic characters terminated by <SP> if parameters follow
  1178. and <CRLF> otherwise. The syntax of mailboxes must conform to
  1179. receiver site conventions. The SMTP commands are discussed
  1180. below. The SMTP replies are discussed in the Section 4.2.
  1181.  
  1182. A mail transaction involves several data objects which are
  1183. communicated as arguments to different commands. The
  1184. reverse-path is the argument of the MAIL command, the
  1185. forward-path is the argument of the RCPT command, and the mail
  1186. data is the argument of the DATA command. These arguments or
  1187. data objects must be transmitted and held pending the
  1188. confirmation communicated by the end of mail data indication
  1189. which finalizes the transaction. The model for this is that
  1190. distinct buffers are provided to hold the types of data
  1191. objects, that is, there is a reverse-path buffer, a
  1192. forward-path buffer, and a mail data buffer. Specific commands
  1193. cause information to be appended to a specific buffer, or cause
  1194. one or more buffers to be cleared.
  1195.  
  1196. HELLO (HELO)
  1197.  
  1198. This command is used to identify the sender-SMTP to the
  1199. receiver-SMTP. The argument field contains the host name of
  1200. the sender-SMTP.
  1201.  
  1202. The receiver-SMTP identifies itself to the sender-SMTP in
  1203. the connection greeting reply, and in the response to this
  1204. command.
  1205.  
  1206. This command and an OK reply to it confirm that both the
  1207. sender-SMTP and the receiver-SMTP are in the initial state,
  1208. that is, there is no transaction in progress and all state
  1209. tables and buffers are cleared.
  1210.  
  1211.  
  1212.  
  1213.  
  1214.  
  1215.  
  1216.  
  1217. Postel [Page 19]
  1218.  
  1219.  
  1220. August 1982 RFC 821
  1221. Simple Mail Transfer Protocol
  1222.  
  1223.  
  1224.  
  1225. MAIL (MAIL)
  1226.  
  1227. This command is used to initiate a mail transaction in which
  1228. the mail data is delivered to one or more mailboxes. The
  1229. argument field contains a reverse-path.
  1230.  
  1231. The reverse-path consists of an optional list of hosts and
  1232. the sender mailbox. When the list of hosts is present, it
  1233. is a "reverse" source route and indicates that the mail was
  1234. relayed through each host on the list (the first host in the
  1235. list was the most recent relay). This list is used as a
  1236. source route to return non-delivery notices to the sender.
  1237. As each relay host adds itself to the beginning of the list,
  1238. it must use its name as known in the IPCE to which it is
  1239. relaying the mail rather than the IPCE from which the mail
  1240. came (if they are different). In some types of error
  1241. reporting messages (for example, undeliverable mail
  1242. notifications) the reverse-path may be null (see Example 7).
  1243.  
  1244. This command clears the reverse-path buffer, the
  1245. forward-path buffer, and the mail data buffer; and inserts
  1246. the reverse-path information from this command into the
  1247. reverse-path buffer.
  1248.  
  1249. RECIPIENT (RCPT)
  1250.  
  1251. This command is used to identify an individual recipient of
  1252. the mail data; multiple recipients are specified by multiple
  1253. use of this command.
  1254.  
  1255. The forward-path consists of an optional list of hosts and a
  1256. required destination mailbox. When the list of hosts is
  1257. present, it is a source route and indicates that the mail
  1258. must be relayed to the next host on the list. If the
  1259. receiver-SMTP does not implement the relay function it may
  1260. user the same reply it would for an unknown local user
  1261. (550).
  1262.  
  1263. When mail is relayed, the relay host must remove itself from
  1264. the beginning forward-path and put itself at the beginning
  1265. of the reverse-path. When mail reaches its ultimate
  1266. destination (the forward-path contains only a destination
  1267. mailbox), the receiver-SMTP inserts it into the destination
  1268. mailbox in accordance with its host mail conventions.
  1269.  
  1270.  
  1271.  
  1272.  
  1273.  
  1274. [Page 20] Postel
  1275.  
  1276.  
  1277.  
  1278.  
  1279. RFC 821 August 1982
  1280. Simple Mail Transfer Protocol
  1281.  
  1282.  
  1283.  
  1284. For example, mail received at relay host A with arguments
  1285.  
  1286. FROM:<USERX@HOSTY.ARPA>
  1287. TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>
  1288.  
  1289. will be relayed on to host B with arguments
  1290.  
  1291. FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
  1292. TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.
  1293.  
  1294. This command causes its forward-path argument to be appended
  1295. to the forward-path buffer.
  1296.  
  1297. DATA (DATA)
  1298.  
  1299. The receiver treats the lines following the command as mail
  1300. data from the sender. This command causes the mail data
  1301. from this command to be appended to the mail data buffer.
  1302. The mail data may contain any of the 128 ASCII character
  1303. codes.
  1304.  
  1305. The mail data is terminated by a line containing only a
  1306. period, that is the character sequence "<CRLF>.<CRLF>" (see
  1307. Section 4.5.2 on Transparency). This is the end of mail
  1308. data indication.
  1309.  
  1310. The end of mail data indication requires that the receiver
  1311. must now process the stored mail transaction information.
  1312. This processing consumes the information in the reverse-path
  1313. buffer, the forward-path buffer, and the mail data buffer,
  1314. and on the completion of this command these buffers are
  1315. cleared. If the processing is successful the receiver must
  1316. send an OK reply. If the processing fails completely the
  1317. receiver must send a failure reply.
  1318.  
  1319. When the receiver-SMTP accepts a message either for relaying
  1320. or for final delivery it inserts at the beginning of the
  1321. mail data a time stamp line. The time stamp line indicates
  1322. the identity of the host that sent the message, and the
  1323. identity of the host that received the message (and is
  1324. inserting this time stamp), and the date and time the
  1325. message was received. Relayed messages will have multiple
  1326. time stamp lines.
  1327.  
  1328. When the receiver-SMTP makes the "final delivery" of a
  1329. message it inserts at the beginning of the mail data a
  1330.  
  1331.  
  1332.  
  1333. Postel [Page 21]
  1334.  
  1335.  
  1336. August 1982 RFC 821
  1337. Simple Mail Transfer Protocol
  1338.  
  1339.  
  1340.  
  1341. return path line. The return path line preserves the
  1342. information in the <reverse-path> from the MAIL command.
  1343. Here, final delivery means the message leaves the SMTP
  1344. world. Normally, this would mean it has been delivered to
  1345. the destination user, but in some cases it may be further
  1346. processed and transmitted by another mail system.
  1347.  
  1348. It is possible for the mailbox in the return path be
  1349. different from the actual sender's mailbox, for example,
  1350. if error responses are to be delivered a special error
  1351. handling mailbox rather than the message senders.
  1352.  
  1353. The preceding two paragraphs imply that the final mail data
  1354. will begin with a return path line, followed by one or more
  1355. time stamp lines. These lines will be followed by the mail
  1356. data header and body [2]. See Example 8.
  1357.  
  1358. Special mention is needed of the response and further action
  1359. required when the processing following the end of mail data
  1360. indication is partially successful. This could arise if
  1361. after accepting several recipients and the mail data, the
  1362. receiver-SMTP finds that the mail data can be successfully
  1363. delivered to some of the recipients, but it cannot be to
  1364. others (for example, due to mailbox space allocation
  1365. problems). In such a situation, the response to the DATA
  1366. command must be an OK reply. But, the receiver-SMTP must
  1367. compose and send an "undeliverable mail" notification
  1368. message to the originator of the message. Either a single
  1369. notification which lists all of the recipients that failed
  1370. to get the message, or separate notification messages must
  1371. be sent for each failed recipient (see Example 7). All
  1372. undeliverable mail notification messages are sent using the
  1373. MAIL command (even if they result from processing a SEND,
  1374. SOML, or SAML command).
  1375.  
  1376.  
  1377.  
  1378.  
  1379.  
  1380.  
  1381.  
  1382.  
  1383.  
  1384.  
  1385.  
  1386.  
  1387.  
  1388.  
  1389.  
  1390. [Page 22] Postel
  1391.  
  1392.  
  1393.  
  1394.  
  1395. RFC 821 August 1982
  1396. Simple Mail Transfer Protocol
  1397.  
  1398.  
  1399.  
  1400. -------------------------------------------------------------
  1401.  
  1402. Example of Return Path and Received Time Stamps
  1403.  
  1404. Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
  1405. Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
  1406. Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
  1407. Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
  1408. Date: 27 Oct 81 15:01:01 PST
  1409. From: JOE@ABC.ARPA
  1410. Subject: Improved Mailing System Installed
  1411. To: SAM@JKL.ARPA
  1412.  
  1413. This is to inform you that ...
  1414.  
  1415. Example 8
  1416.  
  1417. -------------------------------------------------------------
  1418.  
  1419. SEND (SEND)
  1420.  
  1421. This command is used to initiate a mail transaction in which
  1422. the mail data is delivered to one or more terminals. The
  1423. argument field contains a reverse-path. This command is
  1424. successful if the message is delivered to a terminal.
  1425.  
  1426. The reverse-path consists of an optional list of hosts and
  1427. the sender mailbox. When the list of hosts is present, it
  1428. is a "reverse" source route and indicates that the mail was
  1429. relayed through each host on the list (the first host in the
  1430. list was the most recent relay). This list is used as a
  1431. source route to return non-delivery notices to the sender.
  1432. As each relay host adds itself to the beginning of the list,
  1433. it must use its name as known in the IPCE to which it is
  1434. relaying the mail rather than the IPCE from which the mail
  1435. came (if they are different).
  1436.  
  1437. This command clears the reverse-path buffer, the
  1438. forward-path buffer, and the mail data buffer; and inserts
  1439. the reverse-path information from this command into the
  1440. reverse-path buffer.
  1441.  
  1442. SEND OR MAIL (SOML)
  1443.  
  1444. This command is used to initiate a mail transaction in which
  1445. the mail data is delivered to one or more terminals or
  1446.  
  1447.  
  1448.  
  1449. Postel [Page 23]
  1450.  
  1451.  
  1452. August 1982 RFC 821
  1453. Simple Mail Transfer Protocol
  1454.  
  1455.  
  1456.  
  1457. mailboxes. For each recipient the mail data is delivered to
  1458. the recipient's terminal if the recipient is active on the
  1459. host (and accepting terminal messages), otherwise to the
  1460. recipient's mailbox. The argument field contains a
  1461. reverse-path. This command is successful if the message is
  1462. delivered to a terminal or the mailbox.
  1463.  
  1464. The reverse-path consists of an optional list of hosts and
  1465. the sender mailbox. When the list of hosts is present, it
  1466. is a "reverse" source route and indicates that the mail was
  1467. relayed through each host on the list (the first host in the
  1468. list was the most recent relay). This list is used as a
  1469. source route to return non-delivery notices to the sender.
  1470. As each relay host adds itself to the beginning of the list,
  1471. it must use its name as known in the IPCE to which it is
  1472. relaying the mail rather than the IPCE from which the mail
  1473. came (if they are different).
  1474.  
  1475. This command clears the reverse-path buffer, the
  1476. forward-path buffer, and the mail data buffer; and inserts
  1477. the reverse-path information from this command into the
  1478. reverse-path buffer.
  1479.  
  1480. SEND AND MAIL (SAML)
  1481.  
  1482. This command is used to initiate a mail transaction in which
  1483. the mail data is delivered to one or more terminals and
  1484. mailboxes. For each recipient the mail data is delivered to
  1485. the recipient's terminal if the recipient is active on the
  1486. host (and accepting terminal messages), and for all
  1487. recipients to the recipient's mailbox. The argument field
  1488. contains a reverse-path. This command is successful if the
  1489. message is delivered to the mailbox.
  1490.  
  1491. The reverse-path consists of an optional list of hosts and
  1492. the sender mailbox. When the list of hosts is present, it
  1493. is a "reverse" source route and indicates that the mail was
  1494. relayed through each host on the list (the first host in the
  1495. list was the most recent relay). This list is used as a
  1496. source route to return non-delivery notices to the sender.
  1497. As each relay host adds itself to the beginning of the list,
  1498. it must use its name as known in the IPCE to which it is
  1499. relaying the mail rather than the IPCE from which the mail
  1500. came (if they are different).
  1501.  
  1502. This command clears the reverse-path buffer, the
  1503.  
  1504.  
  1505.  
  1506. [Page 24] Postel
  1507.  
  1508.  
  1509.  
  1510.  
  1511. RFC 821 August 1982
  1512. Simple Mail Transfer Protocol
  1513.  
  1514.  
  1515.  
  1516. forward-path buffer, and the mail data buffer; and inserts
  1517. the reverse-path information from this command into the
  1518. reverse-path buffer.
  1519.  
  1520. RESET (RSET)
  1521.  
  1522. This command specifies that the current mail transaction is
  1523. to be aborted. Any stored sender, recipients, and mail data
  1524. must be discarded, and all buffers and state tables cleared.
  1525. The receiver must send an OK reply.
  1526.  
  1527. VERIFY (VRFY)
  1528.  
  1529. This command asks the receiver to confirm that the argument
  1530. identifies a user. If it is a user name, the full name of
  1531. the user (if known) and the fully specified mailbox are
  1532. returned.
  1533.  
  1534. This command has no effect on any of the reverse-path
  1535. buffer, the forward-path buffer, or the mail data buffer.
  1536.  
  1537. EXPAND (EXPN)
  1538.  
  1539. This command asks the receiver to confirm that the argument
  1540. identifies a mailing list, and if so, to return the
  1541. membership of that list. The full name of the users (if
  1542. known) and the fully specified mailboxes are returned in a
  1543. multiline reply.
  1544.  
  1545. This command has no effect on any of the reverse-path
  1546. buffer, the forward-path buffer, or the mail data buffer.
  1547.  
  1548. HELP (HELP)
  1549.  
  1550. This command causes the receiver to send helpful information
  1551. to the sender of the HELP command. The command may take an
  1552. argument (e.g., any command name) and return more specific
  1553. information as a response.
  1554.  
  1555. This command has no effect on any of the reverse-path
  1556. buffer, the forward-path buffer, or the mail data buffer.
  1557.  
  1558.  
  1559.  
  1560.  
  1561.  
  1562.  
  1563.  
  1564.  
  1565. Postel [Page 25]
  1566.  
  1567.  
  1568. August 1982 RFC 821
  1569. Simple Mail Transfer Protocol
  1570.  
  1571.  
  1572.  
  1573. NOOP (NOOP)
  1574.  
  1575. This command does not affect any parameters or previously
  1576. entered commands. It specifies no action other than that
  1577. the receiver send an OK reply.
  1578.  
  1579. This command has no effect on any of the reverse-path
  1580. buffer, the forward-path buffer, or the mail data buffer.
  1581.  
  1582. QUIT (QUIT)
  1583.  
  1584. This command specifies that the receiver must send an OK
  1585. reply, and then close the transmission channel.
  1586.  
  1587. The receiver should not close the transmission channel until
  1588. it receives and replies to a QUIT command (even if there was
  1589. an error). The sender should not close the transmission
  1590. channel until it send a QUIT command and receives the reply
  1591. (even if there was an error response to a previous command).
  1592. If the connection is closed prematurely the receiver should
  1593. act as if a RSET command had been received (canceling any
  1594. pending transaction, but not undoing any previously
  1595. completed transaction), the sender should act as if the
  1596. command or transaction in progress had received a temporary
  1597. error (4xx).
  1598.  
  1599. TURN (TURN)
  1600.  
  1601. This command specifies that the receiver must either (1)
  1602. send an OK reply and then take on the role of the
  1603. sender-SMTP, or (2) send a refusal reply and retain the role
  1604. of the receiver-SMTP.
  1605.  
  1606. If program-A is currently the sender-SMTP and it sends the
  1607. TURN command and receives an OK reply (250) then program-A
  1608. becomes the receiver-SMTP. Program-A is then in the initial
  1609. state as if the transmission channel just opened, and it
  1610. then sends the 220 service ready greeting.
  1611.  
  1612. If program-B is currently the receiver-SMTP and it receives
  1613. the TURN command and sends an OK reply (250) then program-B
  1614. becomes the sender-SMTP. Program-B is then in the initial
  1615. state as if the transmission channel just opened, and it
  1616. then expects to receive the 220 service ready greeting.
  1617.  
  1618. To refuse to change roles the receiver sends the 502 reply.
  1619.  
  1620.  
  1621.  
  1622. [Page 26] Postel
  1623.  
  1624.  
  1625.  
  1626.  
  1627. RFC 821 August 1982
  1628. Simple Mail Transfer Protocol
  1629.  
  1630.  
  1631.  
  1632. There are restrictions on the order in which these command may
  1633. be used.
  1634.  
  1635. The first command in a session must be the HELO command.
  1636. The HELO command may be used later in a session as well. If
  1637. the HELO command argument is not acceptable a 501 failure
  1638. reply must be returned and the receiver-SMTP must stay in
  1639. the same state.
  1640.  
  1641. The NOOP, HELP, EXPN, and VRFY commands can be used at any
  1642. time during a session.
  1643.  
  1644. The MAIL, SEND, SOML, or SAML commands begin a mail
  1645. transaction. Once started a mail transaction consists of
  1646. one of the transaction beginning commands, one or more RCPT
  1647. commands, and a DATA command, in that order. A mail
  1648. transaction may be aborted by the RSET command. There may
  1649. be zero or more transactions in a session.
  1650.  
  1651. If the transaction beginning command argument is not
  1652. acceptable a 501 failure reply must be returned and the
  1653. receiver-SMTP must stay in the same state. If the commands
  1654. in a transaction are out of order a 503 failure reply must
  1655. be returned and the receiver-SMTP must stay in the same
  1656. state.
  1657.  
  1658. The last command in a session must be the QUIT command. The
  1659. QUIT command can not be used at any other time in a session.
  1660.  
  1661. 4.1.2. COMMAND SYNTAX
  1662.  
  1663. The commands consist of a command code followed by an argument
  1664. field. Command codes are four alphabetic characters. Upper
  1665. and lower case alphabetic characters are to be treated
  1666. identically. Thus, any of the following may represent the mail
  1667. command:
  1668.  
  1669. MAIL Mail mail MaIl mAIl
  1670.  
  1671. This also applies to any symbols representing parameter values,
  1672. such as "TO" or "to" for the forward-path. Command codes and
  1673. the argument fields are separated by one or more spaces.
  1674. However, within the reverse-path and forward-path arguments
  1675. case is important. In particular, in some hosts the user
  1676. "smith" is different from the user "Smith".
  1677.  
  1678.  
  1679.  
  1680.  
  1681. Postel [Page 27]
  1682.  
  1683.  
  1684. August 1982 RFC 821
  1685. Simple Mail Transfer Protocol
  1686.  
  1687.  
  1688.  
  1689. The argument field consists of a variable length character
  1690. string ending with the character sequence <CRLF>. The receiver
  1691. is to take no action until this sequence is received.
  1692.  
  1693. Square brackets denote an optional argument field. If the
  1694. option is not taken, the appropriate default is implied.
  1695.  
  1696.  
  1697.  
  1698.  
  1699.  
  1700.  
  1701.  
  1702.  
  1703.  
  1704.  
  1705.  
  1706.  
  1707.  
  1708.  
  1709.  
  1710.  
  1711.  
  1712.  
  1713.  
  1714.  
  1715.  
  1716.  
  1717.  
  1718.  
  1719.  
  1720.  
  1721.  
  1722.  
  1723.  
  1724.  
  1725.  
  1726.  
  1727.  
  1728.  
  1729.  
  1730.  
  1731.  
  1732.  
  1733.  
  1734.  
  1735.  
  1736.  
  1737.  
  1738. [Page 28] Postel
  1739.  
  1740.  
  1741.  
  1742.  
  1743. RFC 821 August 1982
  1744. Simple Mail Transfer Protocol
  1745.  
  1746.  
  1747.  
  1748. The following are the SMTP commands:
  1749.  
  1750. HELO <SP> <domain> <CRLF>
  1751.  
  1752. MAIL <SP> FROM:<reverse-path> <CRLF>
  1753.  
  1754. RCPT <SP> TO:<forward-path> <CRLF>
  1755.  
  1756. DATA <CRLF>
  1757.  
  1758. RSET <CRLF>
  1759.  
  1760. SEND <SP> FROM:<reverse-path> <CRLF>
  1761.  
  1762. SOML <SP> FROM:<reverse-path> <CRLF>
  1763.  
  1764. SAML <SP> FROM:<reverse-path> <CRLF>
  1765.  
  1766. VRFY <SP> <string> <CRLF>
  1767.  
  1768. EXPN <SP> <string> <CRLF>
  1769.  
  1770. HELP [<SP> <string>] <CRLF>
  1771.  
  1772. NOOP <CRLF>
  1773.  
  1774. QUIT <CRLF>
  1775.  
  1776. TURN <CRLF>
  1777.  
  1778.  
  1779.  
  1780.  
  1781.  
  1782.  
  1783.  
  1784.  
  1785.  
  1786.  
  1787.  
  1788.  
  1789.  
  1790.  
  1791.  
  1792.  
  1793.  
  1794.  
  1795.  
  1796.  
  1797. Postel [Page 29]
  1798.  
  1799.  
  1800. August 1982 RFC 821
  1801. Simple Mail Transfer Protocol
  1802.  
  1803.  
  1804.  
  1805. The syntax of the above argument fields (using BNF notation
  1806. where applicable) is given below. The "..." notation indicates
  1807. that a field may be repeated one or more times.
  1808.  
  1809. <reverse-path> ::= <path>
  1810.  
  1811. <forward-path> ::= <path>
  1812.  
  1813. <path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
  1814.  
  1815. <a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
  1816.  
  1817. <at-domain> ::= "@" <domain>
  1818.  
  1819. <domain> ::= <element> | <element> "." <domain>
  1820.  
  1821. <element> ::= <name> | "#" <number> | "[" <dotnum> "]"
  1822.  
  1823. <mailbox> ::= <local-part> "@" <domain>
  1824.  
  1825. <local-part> ::= <dot-string> | <quoted-string>
  1826.  
  1827. <name> ::= <a> <ldh-str> <let-dig>
  1828.  
  1829. <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
  1830.  
  1831. <let-dig> ::= <a> | <d>
  1832.  
  1833. <let-dig-hyp> ::= <a> | <d> | "-"
  1834.  
  1835. <dot-string> ::= <string> | <string> "." <dot-string>
  1836.  
  1837. <string> ::= <char> | <char> <string>
  1838.  
  1839. <quoted-string> ::= """ <qtext> """
  1840.  
  1841. <qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
  1842.  
  1843. <char> ::= <c> | "\" <x>
  1844.  
  1845. <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
  1846.  
  1847. <number> ::= <d> | <d> <number>
  1848.  
  1849. <CRLF> ::= <CR> <LF>
  1850.  
  1851.  
  1852.  
  1853.  
  1854. [Page 30] Postel
  1855.  
  1856.  
  1857.  
  1858.  
  1859. RFC 821 August 1982
  1860. Simple Mail Transfer Protocol
  1861.  
  1862.  
  1863.  
  1864. <CR> ::= the carriage return character (ASCII code 13)
  1865.  
  1866. <LF> ::= the line feed character (ASCII code 10)
  1867.  
  1868. <SP> ::= the space character (ASCII code 32)
  1869.  
  1870. <snum> ::= one, two, or three digits representing a decimal
  1871. integer value in the range 0 through 255
  1872.  
  1873. <a> ::= any one of the 52 alphabetic characters A through Z
  1874. in upper case and a through z in lower case
  1875.  
  1876. <c> ::= any one of the 128 ASCII characters, but not any
  1877. <special> or <SP>
  1878.  
  1879. <d> ::= any one of the ten digits 0 through 9
  1880.  
  1881. <q> ::= any one of the 128 ASCII characters except <CR>,
  1882. <LF>, quote ("), or backslash (\)
  1883.  
  1884. <x> ::= any one of the 128 ASCII characters (no exceptions)
  1885.  
  1886. <special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
  1887. | "," | ";" | ":" | "@" """ | the control
  1888. characters (ASCII codes 0 through 31 inclusive and
  1889. 127)
  1890.  
  1891. Note that the backslash, "\", is a quote character, which is
  1892. used to indicate that the next character is to be used
  1893. literally (instead of its normal interpretation). For example,
  1894. "Joe\,Smith" could be used to indicate a single nine character
  1895. user field with comma being the fourth character of the field.
  1896.  
  1897. Hosts are generally known by names which are translated to
  1898. addresses in each host. Note that the name elements of domains
  1899. are the official names -- no use of nicknames or aliases is
  1900. allowed.
  1901.  
  1902. Sometimes a host is not known to the translation function and
  1903. communication is blocked. To bypass this barrier two numeric
  1904. forms are also allowed for host "names". One form is a decimal
  1905. integer prefixed by a pound sign, "#", which indicates the
  1906. number is the address of the host. Another form is four small
  1907. decimal integers separated by dots and enclosed by brackets,
  1908. e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
  1909. Address in four 8-bit fields.
  1910.  
  1911.  
  1912.  
  1913. Postel [Page 31]
  1914.  
  1915.  
  1916. August 1982 RFC 821
  1917. Simple Mail Transfer Protocol
  1918.  
  1919.  
  1920.  
  1921. The time stamp line and the return path line are formally
  1922. defined as follows:
  1923.  
  1924. <return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
  1925.  
  1926. <time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
  1927.  
  1928. <stamp> ::= <from-domain> <by-domain> <opt-info> ";"
  1929. <daytime>
  1930.  
  1931. <from-domain> ::= "FROM" <SP> <domain> <SP>
  1932.  
  1933. <by-domain> ::= "BY" <SP> <domain> <SP>
  1934.  
  1935. <opt-info> ::= [<via>] [<with>] [<id>] [<for>]
  1936.  
  1937. <via> ::= "VIA" <SP> <link> <SP>
  1938.  
  1939. <with> ::= "WITH" <SP> <protocol> <SP>
  1940.  
  1941. <id> ::= "ID" <SP> <string> <SP>
  1942.  
  1943. <for> ::= "FOR" <SP> <path> <SP>
  1944.  
  1945. <link> ::= The standard names for links are registered with
  1946. the Network Information Center.
  1947.  
  1948. <protocol> ::= The standard names for protocols are
  1949. registered with the Network Information Center.
  1950.  
  1951. <daytime> ::= <SP> <date> <SP> <time>
  1952.  
  1953. <date> ::= <dd> <SP> <mon> <SP> <yy>
  1954.  
  1955. <time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
  1956.  
  1957. <dd> ::= the one or two decimal integer day of the month in
  1958. the range 1 to 31.
  1959.  
  1960. <mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
  1961. "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
  1962.  
  1963. <yy> ::= the two decimal integer year of the century in the
  1964. range 00 to 99.
  1965.  
  1966.  
  1967.  
  1968.  
  1969.  
  1970. [Page 32] Postel
  1971.  
  1972.  
  1973.  
  1974.  
  1975. RFC 821 August 1982
  1976. Simple Mail Transfer Protocol
  1977.  
  1978.  
  1979.  
  1980. <hh> ::= the two decimal integer hour of the day in the
  1981. range 00 to 24.
  1982.  
  1983. <mm> ::= the two decimal integer minute of the hour in the
  1984. range 00 to 59.
  1985.  
  1986. <ss> ::= the two decimal integer second of the minute in the
  1987. range 00 to 59.
  1988.  
  1989. <zone> ::= "UT" for Universal Time (the default) or other
  1990. time zone designator (as in [2]).
  1991.  
  1992.  
  1993.  
  1994. -------------------------------------------------------------
  1995.  
  1996. Return Path Example
  1997.  
  1998. Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
  1999.  
  2000. Example 9
  2001.  
  2002. -------------------------------------------------------------
  2003.  
  2004. -------------------------------------------------------------
  2005.  
  2006. Time Stamp Line Example
  2007.  
  2008. Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
  2009.  
  2010. Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
  2011. id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
  2012.  
  2013. Example 10
  2014.  
  2015. -------------------------------------------------------------
  2016.  
  2017.  
  2018.  
  2019.  
  2020.  
  2021.  
  2022.  
  2023.  
  2024.  
  2025.  
  2026.  
  2027.  
  2028.  
  2029. Postel [Page 33]
  2030.  
  2031.  
  2032. August 1982 RFC 821
  2033. Simple Mail Transfer Protocol
  2034.  
  2035.  
  2036.  
  2037. 4.2. SMTP REPLIES
  2038.  
  2039. Replies to SMTP commands are devised to ensure the synchronization
  2040. of requests and actions in the process of mail transfer, and to
  2041. guarantee that the sender-SMTP always knows the state of the
  2042. receiver-SMTP. Every command must generate exactly one reply.
  2043.  
  2044. The details of the command-reply sequence are made explicit in
  2045. Section 5.3 on Sequencing and Section 5.4 State Diagrams.
  2046.  
  2047. An SMTP reply consists of a three digit number (transmitted as
  2048. three alphanumeric characters) followed by some text. The number
  2049. is intended for use by automata to determine what state to enter
  2050. next; the text is meant for the human user. It is intended that
  2051. the three digits contain enough encoded information that the
  2052. sender-SMTP need not examine the text and may either discard it or
  2053. pass it on to the user, as appropriate. In particular, the text
  2054. may be receiver-dependent and context dependent, so there are
  2055. likely to be varying texts for each reply code. A discussion of
  2056. the theory of reply codes is given in Appendix E. Formally, a
  2057. reply is defined to be the sequence: a three-digit code, <SP>,
  2058. one line of text, and <CRLF>, or a multiline reply (as defined in
  2059. Appendix E). Only the EXPN and HELP commands are expected to
  2060. result in multiline replies in normal circumstances, however
  2061. multiline replies are allowed for any command.
  2062.  
  2063.  
  2064.  
  2065.  
  2066.  
  2067.  
  2068.  
  2069.  
  2070.  
  2071.  
  2072.  
  2073.  
  2074.  
  2075.  
  2076.  
  2077.  
  2078.  
  2079.  
  2080.  
  2081.  
  2082.  
  2083.  
  2084.  
  2085.  
  2086. [Page 34] Postel
  2087.  
  2088.  
  2089.  
  2090.  
  2091. RFC 821 August 1982
  2092. Simple Mail Transfer Protocol
  2093.  
  2094.  
  2095.  
  2096. 4.2.1. REPLY CODES BY FUNCTION GROUPS
  2097.  
  2098. 500 Syntax error, command unrecognized
  2099. [This may include errors such as command line too long]
  2100. 501 Syntax error in parameters or arguments
  2101. 502 Command not implemented
  2102. 503 Bad sequence of commands
  2103. 504 Command parameter not implemented
  2104.  
  2105. 211 System status, or system help reply
  2106. 214 Help message
  2107. [Information on how to use the receiver or the meaning of a
  2108. particular non-standard command; this reply is useful only
  2109. to the human user]
  2110.  
  2111. 220 <domain> Service ready
  2112. 221 <domain> Service closing transmission channel
  2113. 421 <domain> Service not available,
  2114. closing transmission channel
  2115. [This may be a reply to any command if the service knows it
  2116. must shut down]
  2117.  
  2118. 250 Requested mail action okay, completed
  2119. 251 User not local; will forward to <forward-path>
  2120. 450 Requested mail action not taken: mailbox unavailable
  2121. [E.g., mailbox busy]
  2122. 550 Requested action not taken: mailbox unavailable
  2123. [E.g., mailbox not found, no access]
  2124. 451 Requested action aborted: error in processing
  2125. 551 User not local; please try <forward-path>
  2126. 452 Requested action not taken: insufficient system storage
  2127. 552 Requested mail action aborted: exceeded storage allocation
  2128. 553 Requested action not taken: mailbox name not allowed
  2129. [E.g., mailbox syntax incorrect]
  2130. 354 Start mail input; end with <CRLF>.<CRLF>
  2131. 554 Transaction failed
  2132.  
  2133.  
  2134.  
  2135.  
  2136.  
  2137.  
  2138.  
  2139.  
  2140.  
  2141.  
  2142.  
  2143.  
  2144.  
  2145. Postel [Page 35]
  2146.  
  2147.  
  2148. August 1982 RFC 821
  2149. Simple Mail Transfer Protocol
  2150.  
  2151.  
  2152.  
  2153. 4.2.2. NUMERIC ORDER LIST OF REPLY CODES
  2154.  
  2155. 211 System status, or system help reply
  2156. 214 Help message
  2157. [Information on how to use the receiver or the meaning of a
  2158. particular non-standard command; this reply is useful only
  2159. to the human user]
  2160. 220 <domain> Service ready
  2161. 221 <domain> Service closing transmission channel
  2162. 250 Requested mail action okay, completed
  2163. 251 User not local; will forward to <forward-path>
  2164.  
  2165. 354 Start mail input; end with <CRLF>.<CRLF>
  2166.  
  2167. 421 <domain> Service not available,
  2168. closing transmission channel
  2169. [This may be a reply to any command if the service knows it
  2170. must shut down]
  2171. 450 Requested mail action not taken: mailbox unavailable
  2172. [E.g., mailbox busy]
  2173. 451 Requested action aborted: local error in processing
  2174. 452 Requested action not taken: insufficient system storage
  2175.  
  2176. 500 Syntax error, command unrecognized
  2177. [This may include errors such as command line too long]
  2178. 501 Syntax error in parameters or arguments
  2179. 502 Command not implemented
  2180. 503 Bad sequence of commands
  2181. 504 Command parameter not implemented
  2182. 550 Requested action not taken: mailbox unavailable
  2183. [E.g., mailbox not found, no access]
  2184. 551 User not local; please try <forward-path>
  2185. 552 Requested mail action aborted: exceeded storage allocation
  2186. 553 Requested action not taken: mailbox name not allowed
  2187. [E.g., mailbox syntax incorrect]
  2188. 554 Transaction failed
  2189.  
  2190.  
  2191.  
  2192.  
  2193.  
  2194.  
  2195.  
  2196.  
  2197.  
  2198.  
  2199.  
  2200.  
  2201.  
  2202. [Page 36] Postel
  2203.  
  2204.  
  2205.  
  2206.  
  2207. RFC 821 August 1982
  2208. Simple Mail Transfer Protocol
  2209.  
  2210.  
  2211.  
  2212. 4.3. SEQUENCING OF COMMANDS AND REPLIES
  2213.  
  2214. The communication between the sender and receiver is intended to
  2215. be an alternating dialogue, controlled by the sender. As such,
  2216. the sender issues a command and the receiver responds with a
  2217. reply. The sender must wait for this response before sending
  2218. further commands.
  2219.  
  2220. One important reply is the connection greeting. Normally, a
  2221. receiver will send a 220 "Service ready" reply when the connection
  2222. is completed. The sender should wait for this greeting message
  2223. before sending any commands.
  2224.  
  2225. Note: all the greeting type replies have the official name of
  2226. the server host as the first word following the reply code.
  2227.  
  2228. For example,
  2229.  
  2230. 220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
  2231.  
  2232. The table below lists alternative success and failure replies for
  2233. each command. These must be strictly adhered to; a receiver may
  2234. substitute text in the replies, but the meaning and action implied
  2235. by the code numbers and by the specific command reply sequence
  2236. cannot be altered.
  2237.  
  2238. COMMAND-REPLY SEQUENCES
  2239.  
  2240. Each command is listed with its possible replies. The prefixes
  2241. used before the possible replies are "P" for preliminary (not
  2242. used in SMTP), "I" for intermediate, "S" for success, "F" for
  2243. failure, and "E" for error. The 421 reply (service not
  2244. available, closing transmission channel) may be given to any
  2245. command if the SMTP-receiver knows it must shut down. This
  2246. listing forms the basis for the State Diagrams in Section 4.4.
  2247.  
  2248. CONNECTION ESTABLISHMENT
  2249. S: 220
  2250. F: 421
  2251. HELO
  2252. S: 250
  2253. E: 500, 501, 504, 421
  2254. MAIL
  2255. S: 250
  2256. F: 552, 451, 452
  2257. E: 500, 501, 421
  2258.  
  2259.  
  2260.  
  2261. Postel [Page 37]
  2262.  
  2263.  
  2264. August 1982 RFC 821
  2265. Simple Mail Transfer Protocol
  2266.  
  2267.  
  2268.  
  2269. RCPT
  2270. S: 250, 251
  2271. F: 550, 551, 552, 553, 450, 451, 452
  2272. E: 500, 501, 503, 421
  2273. DATA
  2274. I: 354 -> data -> S: 250
  2275. F: 552, 554, 451, 452
  2276. F: 451, 554
  2277. E: 500, 501, 503, 421
  2278. RSET
  2279. S: 250
  2280. E: 500, 501, 504, 421
  2281. SEND
  2282. S: 250
  2283. F: 552, 451, 452
  2284. E: 500, 501, 502, 421
  2285. SOML
  2286. S: 250
  2287. F: 552, 451, 452
  2288. E: 500, 501, 502, 421
  2289. SAML
  2290. S: 250
  2291. F: 552, 451, 452
  2292. E: 500, 501, 502, 421
  2293. VRFY
  2294. S: 250, 251
  2295. F: 550, 551, 553
  2296. E: 500, 501, 502, 504, 421
  2297. EXPN
  2298. S: 250
  2299. F: 550
  2300. E: 500, 501, 502, 504, 421
  2301. HELP
  2302. S: 211, 214
  2303. E: 500, 501, 502, 504, 421
  2304. NOOP
  2305. S: 250
  2306. E: 500, 421
  2307. QUIT
  2308. S: 221
  2309. E: 500
  2310. TURN
  2311. S: 250
  2312. F: 502
  2313. E: 500, 503
  2314.  
  2315.  
  2316.  
  2317.  
  2318. [Page 38] Postel
  2319.  
  2320.  
  2321.  
  2322.  
  2323. RFC 821 August 1982
  2324. Simple Mail Transfer Protocol
  2325.  
  2326.  
  2327.  
  2328. 4.4. STATE DIAGRAMS
  2329.  
  2330. Following are state diagrams for a simple-minded SMTP
  2331. implementation. Only the first digit of the reply codes is used.
  2332. There is one state diagram for each group of SMTP commands. The
  2333. command groupings were determined by constructing a model for each
  2334. command and then collecting together the commands with
  2335. structurally identical models.
  2336.  
  2337. For each command there are three possible outcomes: "success"
  2338. (S), "failure" (F), and "error" (E). In the state diagrams below
  2339. we use the symbol B for "begin", and the symbol W for "wait for
  2340. reply".
  2341.  
  2342. First, the diagram that represents most of the SMTP commands:
  2343.  
  2344.  
  2345. 1,3 +---+
  2346. ----------->| E |
  2347. | +---+
  2348. |
  2349. +---+ cmd +---+ 2 +---+
  2350. | B |---------->| W |---------->| S |
  2351. +---+ +---+ +---+
  2352. |
  2353. | 4,5 +---+
  2354. ----------->| F |
  2355. +---+
  2356.  
  2357.  
  2358. This diagram models the commands:
  2359.  
  2360. HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
  2361. NOOP, QUIT, TURN.
  2362.  
  2363.  
  2364.  
  2365.  
  2366.  
  2367.  
  2368.  
  2369.  
  2370.  
  2371.  
  2372.  
  2373.  
  2374.  
  2375.  
  2376.  
  2377. Postel [Page 39]
  2378.  
  2379.  
  2380. August 1982 RFC 821
  2381. Simple Mail Transfer Protocol
  2382.  
  2383.  
  2384.  
  2385. A more complex diagram models the DATA command:
  2386.  
  2387.  
  2388. +---+ DATA +---+ 1,2 +---+
  2389. | B |---------->| W |-------------------->| E |
  2390. +---+ +---+ ------------>+---+
  2391. 3| |4,5 |
  2392. | | |
  2393. -------------- ----- |
  2394. | | | +---+
  2395. | ---------- -------->| S |
  2396. | | | | +---+
  2397. | | ------------
  2398. | | | |
  2399. V 1,3| |2 |
  2400. +---+ data +---+ --------------->+---+
  2401. | |---------->| W | | F |
  2402. +---+ +---+-------------------->+---+
  2403. 4,5
  2404.  
  2405.  
  2406. Note that the "data" here is a series of lines sent from the
  2407. sender to the receiver with no response expected until the last
  2408. line is sent.
  2409.  
  2410.  
  2411.  
  2412.  
  2413.  
  2414.  
  2415.  
  2416.  
  2417.  
  2418.  
  2419.  
  2420.  
  2421.  
  2422.  
  2423.  
  2424.  
  2425.  
  2426.  
  2427.  
  2428.  
  2429.  
  2430.  
  2431.  
  2432.  
  2433.  
  2434. [Page 40] Postel
  2435.  
  2436.  
  2437.  
  2438.  
  2439. RFC 821 August 1982
  2440. Simple Mail Transfer Protocol
  2441.  
  2442.  
  2443.  
  2444. 4.5. DETAILS
  2445.  
  2446. 4.5.1. MINIMUM IMPLEMENTATION
  2447.  
  2448. In order to make SMTP workable, the following minimum
  2449. implementation is required for all receivers:
  2450.  
  2451. COMMANDS -- HELO
  2452. MAIL
  2453. RCPT
  2454. DATA
  2455. RSET
  2456. NOOP
  2457. QUIT
  2458.  
  2459. 4.5.2. TRANSPARENCY
  2460.  
  2461. Without some provision for data transparency the character
  2462. sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
  2463. by the user. In general, users are not aware of such
  2464. "forbidden" sequences. To allow all user composed text to be
  2465. transmitted transparently the following procedures are used.
  2466.  
  2467. 1. Before sending a line of mail text the sender-SMTP checks
  2468. the first character of the line. If it is a period, one
  2469. additional period is inserted at the beginning of the line.
  2470.  
  2471. 2. When a line of mail text is received by the receiver-SMTP
  2472. it checks the line. If the line is composed of a single
  2473. period it is the end of mail. If the first character is a
  2474. period and there are other characters on the line, the first
  2475. character is deleted.
  2476.  
  2477. The mail data may contain any of the 128 ASCII characters. All
  2478. characters are to be delivered to the recipient's mailbox
  2479. including format effectors and other control characters. If
  2480. the transmission channel provides an 8-bit byte (octets) data
  2481. stream, the 7-bit ASCII codes are transmitted right justified
  2482. in the octets with the high order bits cleared to zero.
  2483.  
  2484. In some systems it may be necessary to transform the data as
  2485. it is received and stored. This may be necessary for hosts
  2486. that use a different character set than ASCII as their local
  2487. character set, or that store data in records rather than
  2488.  
  2489.  
  2490.  
  2491.  
  2492.  
  2493. Postel [Page 41]
  2494.  
  2495.  
  2496. August 1982 RFC 821
  2497. Simple Mail Transfer Protocol
  2498.  
  2499.  
  2500.  
  2501. strings. If such transforms are necessary, they must be
  2502. reversible -- especially if such transforms are applied to
  2503. mail being relayed.
  2504.  
  2505. 4.5.3. SIZES
  2506.  
  2507. There are several objects that have required minimum maximum
  2508. sizes. That is, every implementation must be able to receive
  2509. objects of at least these sizes, but must not send objects
  2510. larger than these sizes.
  2511.  
  2512.  
  2513. ****************************************************
  2514. * *
  2515. * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
  2516. * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
  2517. * OF THESE OBJECTS SHOULD BE USED. *
  2518. * *
  2519. ****************************************************
  2520.  
  2521. user
  2522.  
  2523. The maximum total length of a user name is 64 characters.
  2524.  
  2525. domain
  2526.  
  2527. The maximum total length of a domain name or number is 64
  2528. characters.
  2529.  
  2530. path
  2531.  
  2532. The maximum total length of a reverse-path or
  2533. forward-path is 256 characters (including the punctuation
  2534. and element separators).
  2535.  
  2536. command line
  2537.  
  2538. The maximum total length of a command line including the
  2539. command word and the <CRLF> is 512 characters.
  2540.  
  2541. reply line
  2542.  
  2543. The maximum total length of a reply line including the
  2544. reply code and the <CRLF> is 512 characters.
  2545.  
  2546.  
  2547.  
  2548.  
  2549.  
  2550. [Page 42] Postel
  2551.  
  2552.  
  2553.  
  2554.  
  2555. RFC 821 August 1982
  2556. Simple Mail Transfer Protocol
  2557.  
  2558.  
  2559.  
  2560. text line
  2561.  
  2562. The maximum total length of a text line including the
  2563. <CRLF> is 1000 characters (but not counting the leading
  2564. dot duplicated for transparency).
  2565.  
  2566. recipients buffer
  2567.  
  2568. The maximum total number of recipients that must be
  2569. buffered is 100 recipients.
  2570.  
  2571.  
  2572. ****************************************************
  2573. * *
  2574. * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
  2575. * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
  2576. * OF THESE OBJECTS SHOULD BE USED. *
  2577. * *
  2578. ****************************************************
  2579.  
  2580. Errors due to exceeding these limits may be reported by using
  2581. the reply codes, for example:
  2582.  
  2583. 500 Line too long.
  2584.  
  2585. 501 Path too long
  2586.  
  2587. 552 Too many recipients.
  2588.  
  2589. 552 Too much mail data.
  2590.  
  2591.  
  2592.  
  2593.  
  2594.  
  2595.  
  2596.  
  2597.  
  2598.  
  2599.  
  2600.  
  2601.  
  2602.  
  2603.  
  2604.  
  2605.  
  2606.  
  2607.  
  2608.  
  2609. Postel [Page 43]
  2610.  
  2611.  
  2612. August 1982 RFC 821
  2613. Simple Mail Transfer Protocol
  2614.  
  2615.  
  2616.  
  2617. APPENDIX A
  2618.  
  2619. TCP Transport service
  2620.  
  2621. The Transmission Control Protocol [3] is used in the ARPA
  2622. Internet, and in any network following the US DoD standards for
  2623. internetwork protocols.
  2624.  
  2625. Connection Establishment
  2626.  
  2627. The SMTP transmission channel is a TCP connection established
  2628. between the sender process port U and the receiver process port
  2629. L. This single full duplex connection is used as the
  2630. transmission channel. This protocol is assigned the service
  2631. port 25 (31 octal), that is L=25.
  2632.  
  2633. Data Transfer
  2634.  
  2635. The TCP connection supports the transmission of 8-bit bytes.
  2636. The SMTP data is 7-bit ASCII characters. Each character is
  2637. transmitted as an 8-bit byte with the high-order bit cleared to
  2638. zero.
  2639.  
  2640.  
  2641.  
  2642.  
  2643.  
  2644.  
  2645.  
  2646.  
  2647.  
  2648.  
  2649.  
  2650.  
  2651.  
  2652.  
  2653.  
  2654.  
  2655.  
  2656.  
  2657.  
  2658.  
  2659.  
  2660.  
  2661.  
  2662.  
  2663.  
  2664.  
  2665.  
  2666. [Page 44] Postel
  2667.  
  2668.  
  2669.  
  2670.  
  2671. RFC 821 August 1982
  2672. Simple Mail Transfer Protocol
  2673.  
  2674.  
  2675.  
  2676. APPENDIX B
  2677.  
  2678. NCP Transport service
  2679.  
  2680. The ARPANET Host-to-Host Protocol [4] (implemented by the Network
  2681. Control Program) may be used in the ARPANET.
  2682.  
  2683. Connection Establishment
  2684.  
  2685. The SMTP transmission channel is established via NCP between
  2686. the sender process socket U and receiver process socket L. The
  2687. Initial Connection Protocol [5] is followed resulting in a pair
  2688. of simplex connections. This pair of connections is used as
  2689. the transmission channel. This protocol is assigned the
  2690. contact socket 25 (31 octal), that is L=25.
  2691.  
  2692. Data Transfer
  2693.  
  2694. The NCP data connections are established in 8-bit byte mode.
  2695. The SMTP data is 7-bit ASCII characters. Each character is
  2696. transmitted as an 8-bit byte with the high-order bit cleared to
  2697. zero.
  2698.  
  2699.  
  2700.  
  2701.  
  2702.  
  2703.  
  2704.  
  2705.  
  2706.  
  2707.  
  2708.  
  2709.  
  2710.  
  2711.  
  2712.  
  2713.  
  2714.  
  2715.  
  2716.  
  2717.  
  2718.  
  2719.  
  2720.  
  2721.  
  2722.  
  2723.  
  2724.  
  2725. Postel [Page 45]
  2726.  
  2727.  
  2728. August 1982 RFC 821
  2729. Simple Mail Transfer Protocol
  2730.  
  2731.  
  2732.  
  2733. APPENDIX C
  2734.  
  2735. NITS
  2736.  
  2737. The Network Independent Transport Service [6] may be used.
  2738.  
  2739. Connection Establishment
  2740.  
  2741. The SMTP transmission channel is established via NITS between
  2742. the sender process and receiver process. The sender process
  2743. executes the CONNECT primitive, and the waiting receiver
  2744. process executes the ACCEPT primitive.
  2745.  
  2746. Data Transfer
  2747.  
  2748. The NITS connection supports the transmission of 8-bit bytes.
  2749. The SMTP data is 7-bit ASCII characters. Each character is
  2750. transmitted as an 8-bit byte with the high-order bit cleared to
  2751. zero.
  2752.  
  2753.  
  2754.  
  2755.  
  2756.  
  2757.  
  2758.  
  2759.  
  2760.  
  2761.  
  2762.  
  2763.  
  2764.  
  2765.  
  2766.  
  2767.  
  2768.  
  2769.  
  2770.  
  2771.  
  2772.  
  2773.  
  2774.  
  2775.  
  2776.  
  2777.  
  2778.  
  2779.  
  2780.  
  2781.  
  2782. [Page 46] Postel
  2783.  
  2784.  
  2785.  
  2786.  
  2787. RFC 821 August 1982
  2788. Simple Mail Transfer Protocol
  2789.  
  2790.  
  2791.  
  2792. APPENDIX D
  2793.  
  2794. X.25 Transport service
  2795.  
  2796. It may be possible to use the X.25 service [7] as provided by the
  2797. Public Data Networks directly, however, it is suggested that a
  2798. reliable end-to-end protocol such as TCP be used on top of X.25
  2799. connections.
  2800.  
  2801.  
  2802.  
  2803.  
  2804.  
  2805.  
  2806.  
  2807.  
  2808.  
  2809.  
  2810.  
  2811.  
  2812.  
  2813.  
  2814.  
  2815.  
  2816.  
  2817.  
  2818.  
  2819.  
  2820.  
  2821.  
  2822.  
  2823.  
  2824.  
  2825.  
  2826.  
  2827.  
  2828.  
  2829.  
  2830.  
  2831.  
  2832.  
  2833.  
  2834.  
  2835.  
  2836.  
  2837.  
  2838.  
  2839.  
  2840.  
  2841. Postel [Page 47]
  2842.  
  2843.  
  2844. August 1982 RFC 821
  2845. Simple Mail Transfer Protocol
  2846.  
  2847.  
  2848.  
  2849. APPENDIX E
  2850.  
  2851. Theory of Reply Codes
  2852.  
  2853. The three digits of the reply each have a special significance.
  2854. The first digit denotes whether the response is good, bad or
  2855. incomplete. An unsophisticated sender-SMTP will be able to
  2856. determine its next action (proceed as planned, redo, retrench,
  2857. etc.) by simply examining this first digit. A sender-SMTP that
  2858. wants to know approximately what kind of error occurred (e.g.,
  2859. mail system error, command syntax error) may examine the second
  2860. digit, reserving the third digit for the finest gradation of
  2861. information.
  2862.  
  2863. There are five values for the first digit of the reply code:
  2864.  
  2865. 1yz Positive Preliminary reply
  2866.  
  2867. The command has been accepted, but the requested action
  2868. is being held in abeyance, pending confirmation of the
  2869. information in this reply. The sender-SMTP should send
  2870. another command specifying whether to continue or abort
  2871. the action.
  2872.  
  2873. [Note: SMTP does not have any commands that allow this
  2874. type of reply, and so does not have the continue or
  2875. abort commands.]
  2876.  
  2877. 2yz Positive Completion reply
  2878.  
  2879. The requested action has been successfully completed. A
  2880. new request may be initiated.
  2881.  
  2882. 3yz Positive Intermediate reply
  2883.  
  2884. The command has been accepted, but the requested action
  2885. is being held in abeyance, pending receipt of further
  2886. information. The sender-SMTP should send another command
  2887. specifying this information. This reply is used in
  2888. command sequence groups.
  2889.  
  2890. 4yz Transient Negative Completion reply
  2891.  
  2892. The command was not accepted and the requested action did
  2893. not occur. However, the error condition is temporary and
  2894. the action may be requested again. The sender should
  2895.  
  2896.  
  2897.  
  2898. [Page 48] Postel
  2899.  
  2900.  
  2901.  
  2902.  
  2903. RFC 821 August 1982
  2904. Simple Mail Transfer Protocol
  2905.  
  2906.  
  2907.  
  2908. return to the beginning of the command sequence (if any).
  2909. It is difficult to assign a meaning to "transient" when
  2910. two different sites (receiver- and sender- SMTPs) must
  2911. agree on the interpretation. Each reply in this category
  2912. might have a different time value, but the sender-SMTP is
  2913. encouraged to try again. A rule of thumb to determine if
  2914. a reply fits into the 4yz or the 5yz category (see below)
  2915. is that replies are 4yz if they can be repeated without
  2916. any change in command form or in properties of the sender
  2917. or receiver. (E.g., the command is repeated identically
  2918. and the receiver does not put up a new implementation.)
  2919.  
  2920. 5yz Permanent Negative Completion reply
  2921.  
  2922. The command was not accepted and the requested action did
  2923. not occur. The sender-SMTP is discouraged from repeating
  2924. the exact request (in the same sequence). Even some
  2925. "permanent" error conditions can be corrected, so the
  2926. human user may want to direct the sender-SMTP to
  2927. reinitiate the command sequence by direct action at some
  2928. point in the future (e.g., after the spelling has been
  2929. changed, or the user has altered the account status).
  2930.  
  2931. The second digit encodes responses in specific categories:
  2932.  
  2933. x0z Syntax -- These replies refer to syntax errors,
  2934. syntactically correct commands that don't fit any
  2935. functional category, and unimplemented or superfluous
  2936. commands.
  2937.  
  2938. x1z Information -- These are replies to requests for
  2939. information, such as status or help.
  2940.  
  2941. x2z Connections -- These are replies referring to the
  2942. transmission channel.
  2943.  
  2944. x3z Unspecified as yet.
  2945.  
  2946. x4z Unspecified as yet.
  2947.  
  2948. x5z Mail system -- These replies indicate the status of
  2949. the receiver mail system vis-a-vis the requested
  2950. transfer or other mail system action.
  2951.  
  2952. The third digit gives a finer gradation of meaning in each
  2953. category specified by the second digit. The list of replies
  2954.  
  2955.  
  2956.  
  2957. Postel [Page 49]
  2958.  
  2959.  
  2960. August 1982 RFC 821
  2961. Simple Mail Transfer Protocol
  2962.  
  2963.  
  2964.  
  2965. illustrates this. Each reply text is recommended rather than
  2966. mandatory, and may even change according to the command with
  2967. which it is associated. On the other hand, the reply codes
  2968. must strictly follow the specifications in this section.
  2969. Receiver implementations should not invent new codes for
  2970. slightly different situations from the ones described here, but
  2971. rather adapt codes already defined.
  2972.  
  2973. For example, a command such as NOOP whose successful execution
  2974. does not offer the sender-SMTP any new information will return
  2975. a 250 reply. The response is 502 when the command requests an
  2976. unimplemented non-site-specific action. A refinement of that
  2977. is the 504 reply for a command that is implemented, but that
  2978. requests an unimplemented parameter.
  2979.  
  2980. The reply text may be longer than a single line; in these cases
  2981. the complete text must be marked so the sender-SMTP knows when it
  2982. can stop reading the reply. This requires a special format to
  2983. indicate a multiple line reply.
  2984.  
  2985. The format for multiline replies requires that every line,
  2986. except the last, begin with the reply code, followed
  2987. immediately by a hyphen, "-" (also known as minus), followed by
  2988. text. The last line will begin with the reply code, followed
  2989. immediately by <SP>, optionally some text, and <CRLF>.
  2990.  
  2991. For example:
  2992. 123-First line
  2993. 123-Second line
  2994. 123-234 text beginning with numbers
  2995. 123 The last line
  2996.  
  2997. In many cases the sender-SMTP then simply needs to search for
  2998. the reply code followed by <SP> at the beginning of a line, and
  2999. ignore all preceding lines. In a few cases, there is important
  3000. data for the sender in the reply "text". The sender will know
  3001. these cases from the current context.
  3002.  
  3003.  
  3004.  
  3005.  
  3006.  
  3007.  
  3008.  
  3009.  
  3010.  
  3011.  
  3012.  
  3013.  
  3014. [Page 50] Postel
  3015.  
  3016.  
  3017.  
  3018.  
  3019. RFC 821 August 1982
  3020. Simple Mail Transfer Protocol
  3021.  
  3022.  
  3023.  
  3024. APPENDIX F
  3025.  
  3026. Scenarios
  3027.  
  3028. This section presents complete scenarios of several types of SMTP
  3029. sessions.
  3030.  
  3031. A Typical SMTP Transaction Scenario
  3032.  
  3033. This SMTP example shows mail sent by Smith at host USC-ISIF, to
  3034. Jones, Green, and Brown at host BBN-UNIX. Here we assume that
  3035. host USC-ISIF contacts host BBN-UNIX directly. The mail is
  3036. accepted for Jones and Brown. Green does not have a mailbox at
  3037. host BBN-UNIX.
  3038.  
  3039. -------------------------------------------------------------
  3040.  
  3041. R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
  3042. S: HELO USC-ISIF.ARPA
  3043. R: 250 BBN-UNIX.ARPA
  3044.  
  3045. S: MAIL FROM:<Smith@USC-ISIF.ARPA>
  3046. R: 250 OK
  3047.  
  3048. S: RCPT TO:<Jones@BBN-UNIX.ARPA>
  3049. R: 250 OK
  3050.  
  3051. S: RCPT TO:<Green@BBN-UNIX.ARPA>
  3052. R: 550 No such user here
  3053.  
  3054. S: RCPT TO:<Brown@BBN-UNIX.ARPA>
  3055. R: 250 OK
  3056.  
  3057. S: DATA
  3058. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3059. S: Blah blah blah...
  3060. S: ...etc. etc. etc.
  3061. S: .
  3062. R: 250 OK
  3063.  
  3064. S: QUIT
  3065. R: 221 BBN-UNIX.ARPA Service closing transmission channel
  3066.  
  3067. Scenario 1
  3068.  
  3069. -------------------------------------------------------------
  3070.  
  3071.  
  3072.  
  3073. Postel [Page 51]
  3074.  
  3075.  
  3076. August 1982 RFC 821
  3077. Simple Mail Transfer Protocol
  3078.  
  3079.  
  3080.  
  3081. Aborted SMTP Transaction Scenario
  3082.  
  3083. -------------------------------------------------------------
  3084.  
  3085. R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
  3086. S: HELO ISI-VAXA.ARPA
  3087. R: 250 MIT-Multics.ARPA
  3088.  
  3089. S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
  3090. R: 250 OK
  3091.  
  3092. S: RCPT TO:<Jones@MIT-Multics.ARPA>
  3093. R: 250 OK
  3094.  
  3095. S: RCPT TO:<Green@MIT-Multics.ARPA>
  3096. R: 550 No such user here
  3097.  
  3098. S: RSET
  3099. R: 250 OK
  3100.  
  3101. S: QUIT
  3102. R: 221 MIT-Multics.ARPA Service closing transmission channel
  3103.  
  3104. Scenario 2
  3105.  
  3106. -------------------------------------------------------------
  3107.  
  3108.  
  3109.  
  3110.  
  3111.  
  3112.  
  3113.  
  3114.  
  3115.  
  3116.  
  3117.  
  3118.  
  3119.  
  3120.  
  3121.  
  3122.  
  3123.  
  3124.  
  3125.  
  3126.  
  3127.  
  3128.  
  3129.  
  3130. [Page 52] Postel
  3131.  
  3132.  
  3133.  
  3134.  
  3135. RFC 821 August 1982
  3136. Simple Mail Transfer Protocol
  3137.  
  3138.  
  3139.  
  3140. Relayed Mail Scenario
  3141.  
  3142. -------------------------------------------------------------
  3143.  
  3144. Step 1 -- Source Host to Relay Host
  3145.  
  3146. R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
  3147. S: HELO MIT-AI.ARPA
  3148. R: 250 USC-ISIE.ARPA
  3149.  
  3150. S: MAIL FROM:<JQP@MIT-AI.ARPA>
  3151. R: 250 OK
  3152.  
  3153. S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
  3154. R: 250 OK
  3155.  
  3156. S: DATA
  3157. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3158. S: Date: 2 Nov 81 22:33:44
  3159. S: From: John Q. Public <JQP@MIT-AI.ARPA>
  3160. S: Subject: The Next Meeting of the Board
  3161. S: To: Jones@BBN-Vax.ARPA
  3162. S:
  3163. S: Bill:
  3164. S: The next meeting of the board of directors will be
  3165. S: on Tuesday.
  3166. S: John.
  3167. S: .
  3168. R: 250 OK
  3169.  
  3170. S: QUIT
  3171. R: 221 USC-ISIE.ARPA Service closing transmission channel
  3172.  
  3173.  
  3174.  
  3175.  
  3176.  
  3177.  
  3178.  
  3179.  
  3180.  
  3181.  
  3182.  
  3183.  
  3184.  
  3185.  
  3186.  
  3187.  
  3188.  
  3189. Postel [Page 53]
  3190.  
  3191.  
  3192. August 1982 RFC 821
  3193. Simple Mail Transfer Protocol
  3194.  
  3195.  
  3196.  
  3197. Step 2 -- Relay Host to Destination Host
  3198.  
  3199. R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
  3200. S: HELO USC-ISIE.ARPA
  3201. R: 250 BBN-VAX.ARPA
  3202.  
  3203. S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
  3204. R: 250 OK
  3205.  
  3206. S: RCPT TO:<Jones@BBN-VAX.ARPA>
  3207. R: 250 OK
  3208.  
  3209. S: DATA
  3210. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3211. S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
  3212. 2 Nov 81 22:40:10 UT
  3213. S: Date: 2 Nov 81 22:33:44
  3214. S: From: John Q. Public <JQP@MIT-AI.ARPA>
  3215. S: Subject: The Next Meeting of the Board
  3216. S: To: Jones@BBN-Vax.ARPA
  3217. S:
  3218. S: Bill:
  3219. S: The next meeting of the board of directors will be
  3220. S: on Tuesday.
  3221. S: John.
  3222. S: .
  3223. R: 250 OK
  3224.  
  3225. S: QUIT
  3226. R: 221 USC-ISIE.ARPA Service closing transmission channel
  3227.  
  3228. Scenario 3
  3229.  
  3230. -------------------------------------------------------------
  3231.  
  3232.  
  3233.  
  3234.  
  3235.  
  3236.  
  3237.  
  3238.  
  3239.  
  3240.  
  3241.  
  3242.  
  3243.  
  3244.  
  3245.  
  3246. [Page 54] Postel
  3247.  
  3248.  
  3249.  
  3250.  
  3251. RFC 821 August 1982
  3252. Simple Mail Transfer Protocol
  3253.  
  3254.  
  3255.  
  3256. Verifying and Sending Scenario
  3257.  
  3258. -------------------------------------------------------------
  3259.  
  3260. R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
  3261. S: HELO MIT-MC.ARPA
  3262. R: 250 SU-SCORE.ARPA
  3263.  
  3264. S: VRFY Crispin
  3265. R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
  3266.  
  3267. S: SEND FROM:<EAK@MIT-MC.ARPA>
  3268. R: 250 OK
  3269.  
  3270. S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
  3271. R: 250 OK
  3272.  
  3273. S: DATA
  3274. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3275. S: Blah blah blah...
  3276. S: ...etc. etc. etc.
  3277. S: .
  3278. R: 250 OK
  3279.  
  3280. S: QUIT
  3281. R: 221 SU-SCORE.ARPA Service closing transmission channel
  3282.  
  3283. Scenario 4
  3284.  
  3285. -------------------------------------------------------------
  3286.  
  3287.  
  3288.  
  3289.  
  3290.  
  3291.  
  3292.  
  3293.  
  3294.  
  3295.  
  3296.  
  3297.  
  3298.  
  3299.  
  3300.  
  3301.  
  3302.  
  3303.  
  3304.  
  3305. Postel [Page 55]
  3306.  
  3307.  
  3308. August 1982 RFC 821
  3309. Simple Mail Transfer Protocol
  3310.  
  3311.  
  3312.  
  3313. Sending and Mailing Scenarios
  3314.  
  3315. First the user's name is verified, then an attempt is made to
  3316. send to the user's terminal. When that fails, the messages is
  3317. mailed to the user's mailbox.
  3318.  
  3319. -------------------------------------------------------------
  3320.  
  3321. R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
  3322. S: HELO MIT-MC.ARPA
  3323. R: 250 SU-SCORE.ARPA
  3324.  
  3325. S: VRFY Crispin
  3326. R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
  3327.  
  3328. S: SEND FROM:<EAK@MIT-MC.ARPA>
  3329. R: 250 OK
  3330.  
  3331. S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
  3332. R: 450 User not active now
  3333.  
  3334. S: RSET
  3335. R: 250 OK
  3336.  
  3337. S: MAIL FROM:<EAK@MIT-MC.ARPA>
  3338. R: 250 OK
  3339.  
  3340. S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
  3341. R: 250 OK
  3342.  
  3343. S: DATA
  3344. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3345. S: Blah blah blah...
  3346. S: ...etc. etc. etc.
  3347. S: .
  3348. R: 250 OK
  3349.  
  3350. S: QUIT
  3351. R: 221 SU-SCORE.ARPA Service closing transmission channel
  3352.  
  3353. Scenario 5
  3354.  
  3355. -------------------------------------------------------------
  3356.  
  3357.  
  3358.  
  3359.  
  3360.  
  3361.  
  3362. [Page 56] Postel
  3363.  
  3364.  
  3365.  
  3366.  
  3367. RFC 821 August 1982
  3368. Simple Mail Transfer Protocol
  3369.  
  3370.  
  3371.  
  3372. Doing the preceding scenario more efficiently.
  3373.  
  3374. -------------------------------------------------------------
  3375.  
  3376. R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
  3377. S: HELO MIT-MC.ARPA
  3378. R: 250 SU-SCORE.ARPA
  3379.  
  3380. S: VRFY Crispin
  3381. R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
  3382.  
  3383. S: SOML FROM:<EAK@MIT-MC.ARPA>
  3384. R: 250 OK
  3385.  
  3386. S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
  3387. R: 250 User not active now, so will do mail.
  3388.  
  3389. S: DATA
  3390. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3391. S: Blah blah blah...
  3392. S: ...etc. etc. etc.
  3393. S: .
  3394. R: 250 OK
  3395.  
  3396. S: QUIT
  3397. R: 221 SU-SCORE.ARPA Service closing transmission channel
  3398.  
  3399. Scenario 6
  3400.  
  3401. -------------------------------------------------------------
  3402.  
  3403.  
  3404.  
  3405.  
  3406.  
  3407.  
  3408.  
  3409.  
  3410.  
  3411.  
  3412.  
  3413.  
  3414.  
  3415.  
  3416.  
  3417.  
  3418.  
  3419.  
  3420.  
  3421. Postel [Page 57]
  3422.  
  3423.  
  3424. August 1982 RFC 821
  3425. Simple Mail Transfer Protocol
  3426.  
  3427.  
  3428.  
  3429. Mailing List Scenario
  3430.  
  3431. First each of two mailing lists are expanded in separate sessions
  3432. with different hosts. Then the message is sent to everyone that
  3433. appeared on either list (but no duplicates) via a relay host.
  3434.  
  3435. -------------------------------------------------------------
  3436.  
  3437. Step 1 -- Expanding the First List
  3438.  
  3439. R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
  3440. S: HELO SU-SCORE.ARPA
  3441. R: 250 MIT-AI.ARPA
  3442.  
  3443. S: EXPN Example-People
  3444. R: 250-<ABC@MIT-MC.ARPA>
  3445. R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
  3446. R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
  3447. R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
  3448. R: 250-<joe@foo-unix.ARPA>
  3449. R: 250 <xyz@bar-unix.ARPA>
  3450.  
  3451. S: QUIT
  3452. R: 221 MIT-AI.ARPA Service closing transmission channel
  3453.  
  3454.  
  3455.  
  3456.  
  3457.  
  3458.  
  3459.  
  3460.  
  3461.  
  3462.  
  3463.  
  3464.  
  3465.  
  3466.  
  3467.  
  3468.  
  3469.  
  3470.  
  3471.  
  3472.  
  3473.  
  3474.  
  3475.  
  3476.  
  3477.  
  3478. [Page 58] Postel
  3479.  
  3480.  
  3481.  
  3482.  
  3483. RFC 821 August 1982
  3484. Simple Mail Transfer Protocol
  3485.  
  3486.  
  3487.  
  3488. Step 2 -- Expanding the Second List
  3489.  
  3490. R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
  3491. S: HELO SU-SCORE.ARPA
  3492. R: 250 MIT-MC.ARPA
  3493.  
  3494. S: EXPN Interested-Parties
  3495. R: 250-Al Calico <ABC@MIT-MC.ARPA>
  3496. R: 250-<XYZ@MIT-AI.ARPA>
  3497. R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
  3498. R: 250-<fred@BBN-UNIX.ARPA>
  3499. R: 250 <xyz@bar-unix.ARPA>
  3500.  
  3501. S: QUIT
  3502. R: 221 MIT-MC.ARPA Service closing transmission channel
  3503.  
  3504.  
  3505.  
  3506.  
  3507.  
  3508.  
  3509.  
  3510.  
  3511.  
  3512.  
  3513.  
  3514.  
  3515.  
  3516.  
  3517.  
  3518.  
  3519.  
  3520.  
  3521.  
  3522.  
  3523.  
  3524.  
  3525.  
  3526.  
  3527.  
  3528.  
  3529.  
  3530.  
  3531.  
  3532.  
  3533.  
  3534.  
  3535.  
  3536.  
  3537. Postel [Page 59]
  3538.  
  3539.  
  3540. August 1982 RFC 821
  3541. Simple Mail Transfer Protocol
  3542.  
  3543.  
  3544.  
  3545. Step 3 -- Mailing to All via a Relay Host
  3546.  
  3547. R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
  3548. S: HELO SU-SCORE.ARPA
  3549. R: 250 USC-ISIE.ARPA
  3550.  
  3551. S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
  3552. R: 250 OK
  3553. S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
  3554. R: 250 OK
  3555. S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
  3556. R: 250 OK
  3557. S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
  3558. R: 250 OK
  3559. S: RCPT
  3560. TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
  3561. R: 250 OK
  3562. S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
  3563. R: 250 OK
  3564. S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
  3565. R: 250 OK
  3566. S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
  3567. R: 250 OK
  3568.  
  3569. S: DATA
  3570. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3571. S: Blah blah blah...
  3572. S: ...etc. etc. etc.
  3573. S: .
  3574. R: 250 OK
  3575.  
  3576. S: QUIT
  3577. R: 221 USC-ISIE.ARPA Service closing transmission channel
  3578.  
  3579. Scenario 7
  3580.  
  3581. -------------------------------------------------------------
  3582.  
  3583.  
  3584.  
  3585.  
  3586.  
  3587.  
  3588.  
  3589.  
  3590.  
  3591.  
  3592.  
  3593.  
  3594. [Page 60] Postel
  3595.  
  3596.  
  3597.  
  3598.  
  3599. RFC 821 August 1982
  3600. Simple Mail Transfer Protocol
  3601.  
  3602.  
  3603.  
  3604. Forwarding Scenarios
  3605.  
  3606. -------------------------------------------------------------
  3607.  
  3608. R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
  3609. S: HELO LBL-UNIX.ARPA
  3610. R: 250 USC-ISIF.ARPA
  3611.  
  3612. S: MAIL FROM:<mo@LBL-UNIX.ARPA>
  3613. R: 250 OK
  3614.  
  3615. S: RCPT TO:<fred@USC-ISIF.ARPA>
  3616. R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
  3617.  
  3618. S: DATA
  3619. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3620. S: Blah blah blah...
  3621. S: ...etc. etc. etc.
  3622. S: .
  3623. R: 250 OK
  3624.  
  3625. S: QUIT
  3626. R: 221 USC-ISIF.ARPA Service closing transmission channel
  3627.  
  3628. Scenario 8
  3629.  
  3630. -------------------------------------------------------------
  3631.  
  3632.  
  3633.  
  3634.  
  3635.  
  3636.  
  3637.  
  3638.  
  3639.  
  3640.  
  3641.  
  3642.  
  3643.  
  3644.  
  3645.  
  3646.  
  3647.  
  3648.  
  3649.  
  3650.  
  3651.  
  3652.  
  3653. Postel [Page 61]
  3654.  
  3655.  
  3656. August 1982 RFC 821
  3657. Simple Mail Transfer Protocol
  3658.  
  3659.  
  3660.  
  3661. -------------------------------------------------------------
  3662.  
  3663. Step 1 -- Trying the Mailbox at the First Host
  3664.  
  3665. R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
  3666. S: HELO LBL-UNIX.ARPA
  3667. R: 250 USC-ISIF.ARPA
  3668.  
  3669. S: MAIL FROM:<mo@LBL-UNIX.ARPA>
  3670. R: 250 OK
  3671.  
  3672. S: RCPT TO:<fred@USC-ISIF.ARPA>
  3673. R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
  3674.  
  3675. S: RSET
  3676. R: 250 OK
  3677.  
  3678. S: QUIT
  3679. R: 221 USC-ISIF.ARPA Service closing transmission channel
  3680.  
  3681. Step 2 -- Delivering the Mail at the Second Host
  3682.  
  3683. R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
  3684. S: HELO LBL-UNIX.ARPA
  3685. R: 250 USC-ISI.ARPA
  3686.  
  3687. S: MAIL FROM:<mo@LBL-UNIX.ARPA>
  3688. R: 250 OK
  3689.  
  3690. S: RCPT TO:<Jones@USC-ISI.ARPA>
  3691. R: OK
  3692.  
  3693. S: DATA
  3694. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3695. S: Blah blah blah...
  3696. S: ...etc. etc. etc.
  3697. S: .
  3698. R: 250 OK
  3699.  
  3700. S: QUIT
  3701. R: 221 USC-ISI.ARPA Service closing transmission channel
  3702.  
  3703. Scenario 9
  3704.  
  3705. -------------------------------------------------------------
  3706.  
  3707.  
  3708.  
  3709.  
  3710. [Page 62] Postel
  3711.  
  3712.  
  3713.  
  3714.  
  3715. RFC 821 August 1982
  3716. Simple Mail Transfer Protocol
  3717.  
  3718.  
  3719.  
  3720. Too Many Recipients Scenario
  3721.  
  3722. -------------------------------------------------------------
  3723.  
  3724. R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
  3725. S: HELO USC-ISIF.ARPA
  3726. R: 250 BERKELEY.ARPA
  3727.  
  3728. S: MAIL FROM:<Postel@USC-ISIF.ARPA>
  3729. R: 250 OK
  3730.  
  3731. S: RCPT TO:<fabry@BERKELEY.ARPA>
  3732. R: 250 OK
  3733.  
  3734. S: RCPT TO:<eric@BERKELEY.ARPA>
  3735. R: 552 Recipient storage full, try again in another transaction
  3736.  
  3737. S: DATA
  3738. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3739. S: Blah blah blah...
  3740. S: ...etc. etc. etc.
  3741. S: .
  3742. R: 250 OK
  3743.  
  3744. S: MAIL FROM:<Postel@USC-ISIF.ARPA>
  3745. R: 250 OK
  3746.  
  3747. S: RCPT TO:<eric@BERKELEY.ARPA>
  3748. R: 250 OK
  3749.  
  3750. S: DATA
  3751. R: 354 Start mail input; end with <CRLF>.<CRLF>
  3752. S: Blah blah blah...
  3753. S: ...etc. etc. etc.
  3754. S: .
  3755. R: 250 OK
  3756.  
  3757. S: QUIT
  3758. R: 221 BERKELEY.ARPA Service closing transmission channel
  3759.  
  3760. Scenario 10
  3761.  
  3762. -------------------------------------------------------------
  3763.  
  3764. Note that a real implementation must handle many recipients as
  3765. specified in Section 4.5.3.
  3766.  
  3767.  
  3768.  
  3769. Postel [Page 63]
  3770.  
  3771.  
  3772. August 1982 RFC 821
  3773. Simple Mail Transfer Protocol
  3774.  
  3775.  
  3776.  
  3777. GLOSSARY
  3778.  
  3779. ASCII
  3780.  
  3781. American Standard Code for Information Interchange [1].
  3782.  
  3783. command
  3784.  
  3785. A request for a mail service action sent by the sender-SMTP to the
  3786. receiver-SMTP.
  3787.  
  3788. domain
  3789.  
  3790. The hierarchially structured global character string address of a
  3791. host computer in the mail system.
  3792.  
  3793. end of mail data indication
  3794.  
  3795. A special sequence of characters that indicates the end of the
  3796. mail data. In particular, the five characters carriage return,
  3797. line feed, period, carriage return, line feed, in that order.
  3798.  
  3799. host
  3800.  
  3801. A computer in the internetwork environment on which mailboxes or
  3802. SMTP processes reside.
  3803.  
  3804. line
  3805.  
  3806. A a sequence of ASCII characters ending with a <CRLF>.
  3807.  
  3808. mail data
  3809.  
  3810. A sequence of ASCII characters of arbitrary length, which conforms
  3811. to the standard set in the Standard for the Format of ARPA
  3812. Internet Text Messages (RFC 822 [2]).
  3813.  
  3814. mailbox
  3815.  
  3816. A character string (address) which identifies a user to whom mail
  3817. is to be sent. Mailbox normally consists of the host and user
  3818. specifications. The standard mailbox naming convention is defined
  3819. to be "user@domain". Additionally, the "container" in which mail
  3820. is stored.
  3821.  
  3822.  
  3823.  
  3824.  
  3825.  
  3826. [Page 64] Postel
  3827.  
  3828.  
  3829.  
  3830.  
  3831. RFC 821 August 1982
  3832. Simple Mail Transfer Protocol
  3833.  
  3834.  
  3835.  
  3836. receiver-SMTP process
  3837.  
  3838. A process which transfers mail in cooperation with a sender-SMTP
  3839. process. It waits for a connection to be established via the
  3840. transport service. It receives SMTP commands from the
  3841. sender-SMTP, sends replies, and performs the specified operations.
  3842.  
  3843. reply
  3844.  
  3845. A reply is an acknowledgment (positive or negative) sent from
  3846. receiver to sender via the transmission channel in response to a
  3847. command. The general form of a reply is a completion code
  3848. (including error codes) followed by a text string. The codes are
  3849. for use by programs and the text is usually intended for human
  3850. users.
  3851.  
  3852. sender-SMTP process
  3853.  
  3854. A process which transfers mail in cooperation with a receiver-SMTP
  3855. process. A local language may be used in the user interface
  3856. command/reply dialogue. The sender-SMTP initiates the transport
  3857. service connection. It initiates SMTP commands, receives replies,
  3858. and governs the transfer of mail.
  3859.  
  3860. session
  3861.  
  3862. The set of exchanges that occur while the transmission channel is
  3863. open.
  3864.  
  3865. transaction
  3866.  
  3867. The set of exchanges required for one message to be transmitted
  3868. for one or more recipients.
  3869.  
  3870. transmission channel
  3871.  
  3872. A full-duplex communication path between a sender-SMTP and a
  3873. receiver-SMTP for the exchange of commands, replies, and mail
  3874. text.
  3875.  
  3876. transport service
  3877.  
  3878. Any reliable stream-oriented data communication services. For
  3879. example, NCP, TCP, NITS.
  3880.  
  3881.  
  3882.  
  3883.  
  3884.  
  3885. Postel [Page 65]
  3886.  
  3887.  
  3888. August 1982 RFC 821
  3889. Simple Mail Transfer Protocol
  3890.  
  3891.  
  3892.  
  3893. user
  3894.  
  3895. A human being (or a process on behalf of a human being) wishing to
  3896. obtain mail transfer service. In addition, a recipient of
  3897. computer mail.
  3898.  
  3899. word
  3900.  
  3901. A sequence of printing characters.
  3902.  
  3903. <CRLF>
  3904.  
  3905. The characters carriage return and line feed (in that order).
  3906.  
  3907. <SP>
  3908.  
  3909. The space character.
  3910.  
  3911.  
  3912.  
  3913.  
  3914.  
  3915.  
  3916.  
  3917.  
  3918.  
  3919.  
  3920.  
  3921.  
  3922.  
  3923.  
  3924.  
  3925.  
  3926.  
  3927.  
  3928.  
  3929.  
  3930.  
  3931.  
  3932.  
  3933.  
  3934.  
  3935.  
  3936.  
  3937.  
  3938.  
  3939.  
  3940.  
  3941.  
  3942. [Page 66] Postel
  3943.  
  3944.  
  3945.  
  3946.  
  3947. RFC 821 August 1982
  3948. Simple Mail Transfer Protocol
  3949.  
  3950.  
  3951.  
  3952. REFERENCES
  3953.  
  3954. [1] ASCII
  3955.  
  3956. ASCII, "USA Code for Information Interchange", United States of
  3957. America Standards Institute, X3.4, 1968. Also in: Feinler, E.
  3958. and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
  3959. the Defense Communications Agency by SRI International, Menlo
  3960. Park, California, Revised January 1978.
  3961.  
  3962. [2] RFC 822
  3963.  
  3964. Crocker, D., "Standard for the Format of ARPA Internet Text
  3965. Messages," RFC 822, Department of Electrical Engineering,
  3966. University of Delaware, August 1982.
  3967.  
  3968. [3] TCP
  3969.  
  3970. Postel, J., ed., "Transmission Control Protocol - DARPA Internet
  3971. Program Protocol Specification", RFC 793, USC/Information Sciences
  3972. Institute, NTIS AD Number A111091, September 1981. Also in:
  3973. Feinler, E. and J. Postel, eds., "Internet Protocol Transition
  3974. Workbook", SRI International, Menlo Park, California, March 1982.
  3975.  
  3976. [4] NCP
  3977.  
  3978. McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
  3979. January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
  3980. Protocol Handbook", NIC 7104, for the Defense Communications
  3981. Agency by SRI International, Menlo Park, California, Revised
  3982. January 1978.
  3983.  
  3984. [5] Initial Connection Protocol
  3985.  
  3986. Postel, J., "Official Initial Connection Protocol", NIC 7101,
  3987. 11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
  3988. Protocol Handbook", NIC 7104, for the Defense Communications
  3989. Agency by SRI International, Menlo Park, California, Revised
  3990. January 1978.
  3991.  
  3992. [6] NITS
  3993.  
  3994. PSS/SG3, "A Network Independent Transport Service", Study Group 3,
  3995. The Post Office PSS Users Group, February 1980. Available from
  3996. the DCPU, National Physical Laboratory, Teddington, UK.
  3997.  
  3998.  
  3999.  
  4000.  
  4001. Postel [Page 67]
  4002.  
  4003.  
  4004. August 1982 RFC 821
  4005. Simple Mail Transfer Protocol
  4006.  
  4007.  
  4008.  
  4009. [7] X.25
  4010.  
  4011. CCITT, "Recommendation X.25 - Interface Between Data Terminal
  4012. Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
  4013. Terminals Operating in the Packet Mode on Public Data Networks,"
  4014. CCITT Orange Book, Vol. VIII.2, International Telephone and
  4015. Telegraph Consultative Committee, Geneva, 1976.
  4016.  
  4017.  
  4018.  
  4019.  
  4020.  
  4021.  
  4022.  
  4023.  
  4024.  
  4025.  
  4026.  
  4027.  
  4028.  
  4029.  
  4030.  
  4031.  
  4032.  
  4033.  
  4034.  
  4035.  
  4036.  
  4037.  
  4038.  
  4039.  
  4040.  
  4041.  
  4042.  
  4043.  
  4044.  
  4045.  
  4046.  
  4047.  
  4048.  
  4049.  
  4050.  
  4051.  
  4052.  
  4053.  
  4054.  
  4055.  
  4056.  
  4057.  
  4058. [Page 68] Postel
  4059.  
  4060.  
  4061. Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/
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