Eigenpoll : Spam fighting software : Postgrey 

Postgrey is a Postfix policy server implementing greylisting developed by David Schweikert at the ISG.EE.

When a request for delivery of a mail is received by Postfix via SMTP, the triplet CLIENT_IP / SENDER / RECIPIENT is built. If it is the first time that this triplet is seen, or if the triplet was first seen, less than 5 minutes ago, then the mail gets rejected with a temporary error. Hopefully spammers or viruses will not try again later, as it is however required per RFC.