If your email server or email gateway is behind a Cisco ASA firewall, double-check those default inspection rules. TURN OFF ESMTP Inspection to guarantee that the firewall is not messing up your mail flow. I have seen this problem over and over again. The Cisco inspection rules silently reject some SMTP commands and re-write others just enough to make your email administrator’s life miserable.
show run | include smtp # if you see a rule for "inspect esmtp", remove it from the config # check with "show run" to verify that it's gone for good! # "write mem" to save your changes!!!!