How to check if your SMTP server is an open relay

October 23, 2010 by: Akhthar Parvez

If you’re setting up a Mail server, one of the most important tasks you need to do is ensure it’s not an open relay. Open relay means anyone can send out emails using the Mail Server without any authentication. If your SMTP server is an open relay, you are most likely to be blacklisted by all major spam lists.

So, here’s how you can check whether your SMTP server is an open relay:

telnet sysadminguide.com 25

helo server.sysadminguide.com

mail from: test@sysadminguide.com

rcpt to: test@testdomain.com

DATA

Subject: test
Testing open relay
.

quit

If everything went successfully and you were able to send the email, then you have an open relay. Tune your SMTP server to setup sender verification or SMTP authentication.

However, be noted that if you have already popped emails or sent emails using authentication from the PC you’re testing it, you will likely succeed in sending the emails through the above method and that doesn’t mean you have an open relay.

Filed under: SMTP
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