Here is a sample of the mentioned log entries. They seem to come in distinct waves. Sometimes a bunch of errors, other times smtps, and like below a mix.
Feb 10 10:28:48 VM-name postfix/error[16285]: CDFE824037F: to=thiensu1504@yahoo.com, relay=none, delay=0.02, delays=0.01/0/0/0.01, dsn=4.4.3, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=yahoo.com type=MX: Host not found, try again)
Feb 10 10:28:48 VM-name postfix/pickup[14967]: D1B59240380: uid=1003 from=virgie_craft@domain.com
Feb 10 10:28:48 VM-name postfix/cleanup[16781]: D1B59240380: message-id=20140210182848.D1B59240380@VM-host.com
Feb 10 10:28:48 VM-name postfix/qmgr[14968]: D1B59240380: from=virgie_craft@domain.com, size=828, nrcpt=1 (queue active)
Feb 10 10:28:48 VM-name postfix/error[16365]: D1B59240380: to=thiensu198981@yahoo.com, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=4.4.3, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=yahoo.com type=MX: Host not found, try again)
Feb 10 10:28:48 VM-name postfix/pickup[14967]: D5F53240381: uid=1003 from=virgie_craft@domain.com
Feb 10 10:28:48 VM-name postfix/cleanup[16777]: D5F53240381: message-id=20140210182848.D5F53240381@VM-host.com
Feb 10 10:28:48 VM-name postfix/qmgr[14968]: D5F53240381: from=virgie_craft@domain.com, size=828, nrcpt=1 (queue active)
Feb 10 10:28:48 VM-name postfix/error[16275]: D5F53240381: to=thiensu771969@yahoo.com, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=4.4.3, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=yahoo.com type=MX: Host not found, try again)
Feb 10 10:28:48 VM-name postfix/pickup[14967]: D92F5240386: uid=1003 from=virgie_craft@domain.com
Feb 10 10:28:48 VM-name postfix/cleanup[16781]: D92F5240386: message-id=20140210182848.D92F5240386@VM-host.com
Feb 10 10:28:48 VM-name postfix/qmgr[14968]: D92F5240386: from=virgie_craft@domain.com, size=825, nrcpt=1 (queue active)
Feb 10 10:28:48 VM-name postfix/error[16297]: D92F5240386: to=thiensu@yahoo.com.vn, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=4.4.3, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=yahoo.com.vn type=MX: Host not found, try again)
Feb 10 10:28:48 VM-name postfix/pickup[14967]: DC9602403AE: uid=1003 from=virgie_craft@domain.com
Feb 10 10:28:48 VM-name postfix/cleanup[16777]: DC9602403AE: message-id=20140210182848.DC9602403AE@VM-host.com
Feb 10 10:28:48 VM-name postfix/qmgr[14968]: DC9602403AE: from=virgie_craft@domain.com, size=827, nrcpt=1 (queue active)
Feb 10 10:28:48 VM-name postfix/error[16323]: DC9602403AE: to=thiensu_1641@yahoo.com, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=4.4.3, status=deferred (delivery temporarily suspended: Host or domain name not found. Name service error for name=yahoo.com type=MX: Host not found, try again)
Feb 10 10:28:50 VM-name postfix/postfix-script[16935]: stopping the Postfix mail system
Feb 10 10:28:50 VM-name postfix/master[14966]: terminating on signal 15
Feb 10 10:53:24 VM-name dovecot: imap-login: Aborted login (auth failed, 1 attempts): user=<info.domain-user>, method=PLAIN, rip=74.111.162.67, lip=198.252.68.71