127.0.0.1 localhost myhostname

echo "Subject: sendmail test" | sendmail -v me@gmail.com

ole@mki:~$ echo "Subject: sendmail test" | sendmail -v me@gmail.com
    WARNING: local host name (localhost) is not qualified; see cf/README: WHO AM I?
    me@gmail.com... Connecting to [127.0.0.1] via relay...
    220 localhost ESMTP Sendmail 8.15.2/8.15.2/Debian-8ubuntu1; Mon, 14 Aug 2017 14:28:56 -0500; (No UCE/UBE) logging access from: localhost(OK)-localhost [127.0.0.1]
    >>> EHLO localhost
    250-localhost Hello localhost [127.0.0.1], pleased to meet you
    250-ENHANCEDSTATUSCODES
    250-PIPELINING
    250-EXPN
    250-VERB
    250-8BITMIME
    250-SIZE
    250-DSN
    250-ETRN
    250-AUTH DIGEST-MD5 CRAM-MD5
    250-DELIVERBY
    250 HELP
    >>> VERB
    250 2.0.0 Verbose mode
    >>> MAIL From:<ole@localhost> SIZE=23 AUTH=ole@localhost
    250 2.1.0 <ole@localhost>... Sender ok
    >>> RCPT To:<me@gmail.com>
    >>> DATA
    250 2.1.5 <ole.ersoy@gmail.com>... Recipient ok
    354 Enter mail, end with "." on a line by itself
    >>> .