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. It has been reported as efficient and easy to setup
by a large number of system administrators.