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.