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Quick check for an open mail relay

If you're like me, you are getting an ever-increasing flow of spam into your mailbox. What you might not realize, however, if you're a Unix or Linux system administrator, is that you might be part of the problem! In general terms, electronic mail servers (so called "mail transport agents" or MTAs) are only intended to accept mail either to be directly delivered to someone on the system or being sent from somene on the system.

Dave's Answer:

With the expansion of email topographies and more complex configurations, though, most email servers now accept mail from 'trusted sources', other systems that can relay messages through the server to another remote destination. Usually not a big deal, but if you configure things incorrectly, you can leave your system open, making your mail system an open relay, which is very bad: spammers find it and then start pouring their thousands of email messages through your server.

How do you know if your mail server is an open relay?

It turns out that there's a very nice - and very simple - tool to test just this thing. Go to the command line in a shell and type:

telnet relay-test.mail-abuse.org

The results will indicate if you have a problem or not. And if you do have a problem, deal with it immediately! There's lots of good information at openrbl.org to get you started.

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Categorized: Unix and Linux Help (Article 3720)
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