From Mountain Rose Multi Media:
MRFilter is an extreme effective spam and virus filter for the Communigate Pro mail server. MRFilter doesn't try to guess whether a message contains spam, it just know for sure. MRFilter allows you to specify which message is spam, using either your own spam phrases or regular expressions. MRFilter has custom actions for messages where the FROM address is the same as the TO address. MRFilter works on Mac OS X (PPC and Intel), Winows, and Linux. Don't take our word for it, just try the full functional demo yourself.