This tip is for you system admins out there: If you are ever troubleshooting a mail server which is either not accepting or not relaying messages, rather than using the “send and wait” method of testing, telnet directly into the mail server to do your testing.

The linked Microsoft article walks you through the entire process of how to send an email from a telnet session. While the article speaks with regards to Exchange Server, the exact same commands can be applied to any mail server. I have used these exact steps many times to test customer’s mail servers to diagnose problems.

The great thing about using a telnet session is you can use any sender or recipient email address and can immediately see the mail server’s response to each of your commands instead of having to decipher an NDR message.