SPAM is the black plague of the Internet world and this Sendmail milter
attempts to address a small subset of the problem by verifying that the
sender's email address is in good standing by performing an SMTP callback
to the MX server responsible for the sender's domain. This milter is not
intended to be a complete solution to the SPAM plague and so it should be
used in conjunction with other tools such as DNS blacklists and content
filters. Milter-sender also supports grey-listing.

WWW: http://www.snert.com/Software/milter-sender/