Sendmail Confirmation System (SMC) is an Sendmail milter plugin that is
designed to significantly reduce the amount of junk email you receive. It
uses dynamic local and DNS-based remote whitelists, blacklists, and an
original auto-confirmation system for unknown but legitimate senders. It
includes features for protecting recipients from dangerous attachments and
HTML includes. It uses a set of original algorithms, including "Check relay
by NS", which simulates a dynamic whitelisting technique, and "Check delays",
also known as "Greylisting".

WWW: http://milter.sourceforge.net/