http://nemesis.lonestar.org/site/mail_trouble.html

To be RFC 2821 compliant, the system name your computer transmits in the EHLO handshake (this occurs when two systems are about to exchange mail with one another) must be identical to the DNS name of the IP address that your computer is currently attached to. For example, if the DNS name for the IP address your mail server is on says "mailserver2.bobsbox.com", then the EHLO handshake must state that exact name. Just saying "mailserver2" or "bobsbox.com" will not be accepted by a server strictly following the rules. You will need to refer to the documentation for your mail server software to find out how to set this value. The forward DNS "A" record should also agree with the reverse DNS for that IP address.

Depending on the software that a given mail server is running, the error messages that can be returned will vary, but almost all return a 550 response code, followed by a brief explanation.