Greylisting using Milter-greylist
milter-greylist is a mail filter for sendmail that implements grey listing, a spam filtering technique proposed by Evan Harris.
Grey listing works by assuming that contrarily to 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 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. Odds are good that the spammer will send a mail to an honey pot address and get blacklisted in a distributed black list before the second attempt.
This section was re-designed in 1.6.052217 version..