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.