You will learn most about your mail server by talking to it directly in SMTP through a Telnet session. At the same time, you should be monitoring mail logfiles for any messages that occur while you're performing your tests.
In this recipe, we cover a number of techniques for testing and debugging e-mail systems. We will demonstrate how to test your system's ability to:
Receive e-mail by submitting a message directly over SMTP
Send e-mail by sending a message though the mail command
Authenticate users
We'll also mention the location of mail logs, various ways of sending e-mail, debugging SMTP authentication, and other topics. Read on; this recipe should give you a way to find the solution to your problem.