SpamCatcher
I installed a mail server on this box so that everyone could get their mail. The software is called MailEnable. It has many nice features, but does not include anything that limits the amount of spam that comes into an account.
Looking through the unsupported downloads on their site, I noticed that people had written these things called "MTA pickups". Basically, you can tell MailEnable to run every message it receives through one of these pickups for processing. However the message is left when the pickup is done is how it is delivered.