--------------------------------------------------------------------------------------------------------------------------
-- "cat /var/log/mail.log":
--------------------------------------------------------------------------------------------------------------------------

Nov 15 10:19:45 mydebianserver postfix/pickup[19631]: 99DA819C1B8: uid=1000 from=<administrator>
Nov 15 10:19:45 mydebianserver postfix/cleanup[21586]: 99DA819C1B8: message-id=<20111115161945.99DA819C1B8@mydebianserver.localdomain>
Nov 15 10:19:45 mydebianserver postfix/qmgr[19632]: 99DA819C1B8: from=<administrator@mydebianserver.localdomain>, size=334, nrcpt=1 (queue active)
Nov 15 10:19:46 mydebianserver postfix/smtp[21588]: 99DA819C1B8: to=<i_substitudeted_my_real_addr@somedomain_i_own.com>, relay=none, delay=0.45, delays=0.45/0/0/0, dsn=5.4.6, status=bounced (mail for somedomain_i_own.com loops back to myself)
Nov 15 10:19:46 mydebianserver postfix/cleanup[21586]: 0C7E519C1B9: message-id=<20111115161946.0C7E519C1B9@mydebianserver.localdomain>
Nov 15 10:19:46 mydebianserver postfix/bounce[21589]: 99DA819C1B8: sender non-delivery notification: 0C7E519C1B9
Nov 15 10:19:46 mydebianserver postfix/qmgr[19632]: 0C7E519C1B9: from=<>, size=2243, nrcpt=1 (queue active)
Nov 15 10:19:46 mydebianserver postfix/qmgr[19632]: 99DA819C1B8: removed
Nov 15 10:19:46 mydebianserver postfix/local[21590]: 0C7E519C1B9: to=<administrator@mydebianserver.localdomain>, relay=local, delay=0.13, delays=0.07/0/0/0.07, dsn=2.0.0, status=sent (delivered to command: procmail -a "$EXTENSION")
Nov 15 10:19:46 mydebianserver postfix/qmgr[19632]: 0C7E519C1B9: removed

--------------------------------------------------------------------------------------------------------------------------
-- showing the most recent output from "sudo egrep '(warning|error|fatal|panic):' /var/log/mail.log | more" (the others are sure to be fixed already):
--------------------------------------------------------------------------------------------------------------------------
Nov 15 10:19:13 mydebianserver postfix/local[21590]: warning: dict_nis_init: NIS domain name not set - NIS lookups disabled

--------------------------------------------------------------------------------------------------------------------------
-- did "/usr/sbin/sendmail -bv i_substitudeted_my_real_addr@somedomain_i_own.com" and checked <root>'s mail:
--------------------------------------------------------------------------------------------------------------------------
Message XXXX:
From MAILER-DAEMON  Tue Nov 15 10:11:25 2011
X-Original-To: administrator@mydebianserver.localdomain
Date: Tue, 15 Nov 2011 10:11:25 -0600 (CST)
From: MAILER-DAEMON@mydebianserver.localdomain (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: administrator@mydebianserver.localdomain
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
        boundary="7B2CB19C1B8.1321373485/mydebianserver.localdomain"

This is a MIME-encapsulated message.

--7B2CB19C1B8.1321373485/mydebianserver.localdomain
Content-Description: Notification
Content-Type: text/plain; charset=us-ascii

This is the mail system at host mydebianserver.localdomain.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<i_substitudeted_my_real_addr@somedomain_i_own.com>: mail for somedomain_i_own.com loops back to
    myself

--7B2CB19C1B8.1321373485/mydebianserver.localdomain
Content-Description: Delivery report
Content-Type: message/delivery-status

Reporting-MTA: dns; mydebianserver.localdomain
X-Postfix-Queue-ID: 7B2CB19C1B8
X-Postfix-Sender: rfc822; administrator@mydebianserver.localdomain
Arrival-Date: Tue, 15 Nov 2011 10:11:25 -0600 (CST)

Final-Recipient: rfc822; i_substitudeted_my_real_addr@somedomain_i_own.com
Action: failed
Status: 5.4.6
Diagnostic-Code: X-Postfix; mail for somedomain_i_own.com loops back to myself

--7B2CB19C1B8.1321373485/mydebianserver.localdomain
Content-Description: Undelivered Message
Content-Type: message/rfc822

Return-Path: <administrator@mydebianserver.localdomain>
Received: by mydebianserver.localdomain (Postfix, from userid 1000)
        id 7B2CB19C1B8; Tue, 15 Nov 2011 10:11:25 -0600 (CST)
To: i_substitudeted_my_real_addr@somedomain_i_own.com
Message-Id: <20111115161125.7B2CB19C1B8@mydebianserver.localdomain>
Date: Tue, 15 Nov 2011 10:11:25 -0600 (CST)
From: administrator@mydebianserver.localdomain (administrator)

test

--7B2CB19C1B8.1321373485/mydebianserver.localdomain--



--------------------------------------------------------------------------------------------------------------------------
-- showing a bit of /etc/postfix/master.cf (I use port 25000 because port 25 is used by other):
--------------------------------------------------------------------------------------------------------------------------

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
#smtp      inet  n       -       -       -       -       smtpd
25000   inet    n       -       -       -       -       smtpd



--------------------------------------------------------------------------------------------------------------------------
"tcpdump -w file.out -s 0 host somedomain_i_own.com and port 25000":
--------------------------------------------------------------------------------------------------------------------------

tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
^C0 packets captured
4 packets received by filter
0 packets dropped by kernel