While Were on the Subject of SPAM Filters

[Steven Elling]

Here is one that I am particularly interested in:

Tagged Message Delivery Agent (TMDA)

TMDA is an OSI certified software application designed to significantly 
reduce the amount of SPAM/UCE (junk-mail) you receive. As a SPAM filter, 
TMDA combines a "whitelist" (for known/trusted senders), a "blacklist" (for 
undesired senders), and a cryptographically enhanced confirmation system 
(for unknown, but legitimate senders). TMDA strives to be more effectual, 
yet less time-consuming than traditional filters.

http://tmda.net/