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.