Here is tail of mail.log when I use webdnstools to check my domain:
root@mailserver:~# tail -f /var/log/mail.log
Jan 23 15:18:04 mailserver postfix/smtpd[3789]: disconnect from localhost[127.0.0.1]
Jan 23 15:18:04 mailserver postfix/cleanup[3778]: 8087340F21: message-id=<20150123141803.E292F40E0D@mailserver.myexampledomain.org>
Jan 23 15:18:04 mailserver postfix/qmgr[3419]: 8087340F21: from=<amavis@mailserver.myexampledomain.org>, size=1401, nrcpt=1 (queue active)
Jan 23 15:18:04 mailserver postfix/local[3792]: 759F040E26: to=<amavis@mailserver.myexampledomain.org>, relay=local, delay=0.05, delays=0.01/0.03/0/0.01, dsn=2.0.0, status=sent (forwarded as 8087340F21)
Jan 23 15:18:04 mailserver postfix/qmgr[3419]: 759F040E26: removed
Jan 23 15:18:04 mailserver amavis[2466]: (02466-01) Passed CLEAN {RelayedInternal}, MYUSERS <amavis@mailserver.myexampledomain.org> -> <amavis@mailserver.myexampledomain.org>, Message-ID: <20150123141803.E292F40E0D@mailserver.myexampledomain.org>, mail_id: aofD6QfLWJgM, Hits: 0, size: 709, queued_as: 759F040E26, 607 ms
Jan 23 15:18:04 mailserver postfix/smtp[3784]: E292F40E0D: to=<amavis@mailserver.myexampledomain.org>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.68, delays=0.03/0.01/0.02/0.62, dsn=2.0.0, status=sent (250 2.0.0 from MTA(smtp:[127.0.0.1]:10025): 250 2.0.0 Ok: queued as 759F040E26)
Jan 23 15:18:04 mailserver postfix/qmgr[3419]: E292F40E0D: removed
Jan 23 15:18:04 mailserver postfix/pipe[3794]: 8087340F21: to=<postmaster@myexampledomain.org>, relay=dovecot, delay=0.16, delays=0/0.03/0/0.12, dsn=2.0.0, status=sent (delivered via dovecot service)
Jan 23 15:18:04 mailserver postfix/qmgr[3419]: 8087340F21: removed
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: lost connection after CONNECT from unknown[173.255.210.37]
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:29 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:30 mailserver postfix/smtpd[3832]: NOQUEUE: reject: RCPT from unknown[173.255.210.37]: 550 5.1.1 <hostmaster@myexampledomain.org>: Recipient address rejected: User unknown in virtual mailbox table; from=<check@user.com> to=<hostmaster@myexampledomain.org> proto=ESMTP helo=<mail.webdnstools.com>
Jan 23 15:22:30 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: lost connection after CONNECT from unknown[173.255.210.37]
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:32 mailserver postfix/smtpd[3832]: E7F5940DE6: client=unknown[173.255.210.37]
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: lost connection after CONNECT from unknown[173.255.210.37]
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:33 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: 03CFD40DE6: client=unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: lost connection after CONNECT from unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: warning: hostname webdnstools.com does not resolve to address 173.255.210.37
Jan 23 15:22:34 mailserver postfix/smtpd[3832]: connect from unknown[173.255.210.37]
Jan 23 15:22:35 mailserver postfix/smtpd[3832]: NOQUEUE: reject: RCPT from unknown[173.255.210.37]: 550 5.1.1 <abuse@myexampledomain.org>: Recipient address rejected: User unknown in virtual mailbox table; from=<check@user.com> to=<abuse@myexampledomain.org> proto=ESMTP helo=<mail.webdnstools.com>
Jan 23 15:22:35 mailserver postfix/smtpd[3832]: disconnect from unknown[173.255.210.37]