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.
TMDA combines a "whitelist" (for known/trusted senders), a "blacklist"
(for undesired senders), and a cryptographically enhanced confirmation
system (for unknown, but legitimate senders).

WWW: http://tmda.sourceforge.net/