Oh, you have control of the mail server? Can you run SendMail8.9? It has a lot of stuff already built in.

http://maps.vix.com/
(the home of the RBL)
http://www.de.spam.abuse.net/webland/spam/tools/index.html
(all sorts of good stuff)

http://www.iks-jena.de/mitarb/lutz/usenet/teergrube.en.html
(the molasses-in-January anti-spam technique)

http://www.sendmail.org/antispam.html
(the place that wrote the FREE mail program most ISPs use)

http://www.mailshield.com/about.html
(commercial program)

http://pinnacle.primenet.com/abuse/spamblocks
(Good examples of how they block the fake Hotmail spams, etc., with procmail)

If you are going to be rejecting any emails, WARN your users and give them the option of
accepting ALL email if they want. Most people will consider it a valuable service, but a
few will think you are censoring them and gripe.