This tutorial from MacMerc shows how to set up a spare Mac as a spam filtering drone. The idea is that two Macs check the same IMAP account. One (the drone) downloads all the messages and filters them with SpamSieve. The other (your main Mac) gets a spam-free inbox. If a spam message gets through to your main Mac, you can drag it to the “AddToSpam” mailbox to train the drone.