Postgrey is a Postfix policy server implementing greylisting. 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, 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.