Mail Avenger is a highly-configurable, MTA-independent SMTP server
daemon. It lets users run messages through filters like ClamAV and
SpamAssassin during SMTP transactions, so the server can reject mail
before assuming responsibility for its delivery. Other unique features
include TCP SYN fingerprint and network route recording, verification
of sender addresses through SMTP callbacks, SPF (sender policy
framework) as a general policy language, qmail-style control over both
SMTP-level behavior and local delivery of extension addresses,
mail-bomb protection, integration with kernel firewalls, and more.
from David Mazieres (http://www.scs.stanford.edu/~dm/addr/)
