milter-greylist - GreyList milter for Sendmail
milter-greylist is a stand-alone milter written in C that implements the
greylist filtering method, as proposed by Evan Harris.
Grey listing works by assuming that unlike legitimate MTA, spam engines will
not retry sending their junk mail on a temporary error. The filter will
always temporarily reject mail on a first attempt, and to accept it after
some time has elapsed.
If spammers ever try to resend rejected messages, we can assume they will
not stay idle between the two sends (if they do, the spam problem would just
be solved). Odds are good that the spammer will send a mail to an honey pot
address and get blacklisted in several real-time distributed black list
before the second attempt.