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An open SMTP relay is what causes most of the spam you recieve. It is basically a mail server that allows anyone to use it to send e-mail through it, clearly violating RFC 2505.

If you spot one, contact the postmaster or report it on http://www.orbs.org/

If you need to check if your own machine is open, you can telnet to mail-abuse.org which runs a test-script and tells you of the result.