Tagged Message Delivery Agent (TMDA).

TMDA is an OSI certified Python application for qmail systems
designed to significantly reduce the amount of SPAM/UCE you receive.
A "whitelist" filter allows known contacts immediately into your
mailbox, and all others must reply to a simple confirmation to
validate their message.  Once they respond to the confirmation,
their original message is delivered to you.

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