This is a port of Emmanuel Dreyfus' milter-greylist.

Grey listing is a wonderful spam filtering technique, which uses a behavior
trick: spammers never resend a message when they get a temporary error,
whereas real MTA do. The idea is to refuse any mail on first attempt, and
accept it after some time has elapsed.

milter-greylist is a stand-alone milter written in C that implement grey
listing.