ARCHIVED: In Unix, what is an open mail relay?

A computer that functions as an open mail relay can pass along email from anywhere to anywhere else, including messages that are neither from nor destined for its own users. Unscrupulous people who find open mail relays use them to send junk or prank email.

Open relays often occur because a computer's mail transport daemon, usually sendmail, is misconfigured or outdated. In some cases, the versions of sendmail that come with some versions of Unix relay openly by default.

If your computer is configured to allow open relaying, it is essential to fix the problem right away. Spammers constantly scan networks for open relays, and they will almost certainly find yours eventually. When they do, your system may be overloaded with extraneous traffic. You may be blacklisted, so that your computer (and perhaps your institution's central mail relays) will not be able to send mail to the outside world.

Note: Microsoft's Exchange Server is also vulnerable to unauthorized relaying. For more information, see Slipstick Systems' page To prevent SMTP relaying.