Postgrey is a Postfix policy server that
implements greylisting. When a request for mail
delivery 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 10
minutes ago, then the mail gets rejected with a
temporary error. Hopefully, spammers or viruses
will not try again later, even though such
behavior is required per RFC.
