B's Harmaa is Postfix policy server that
implements greylisting and sender policy
framework. What differentiates it from other
similar programs is its unique autowhitelist
feature. When a request for delivery of a message
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 one minute ago,
then the message gets rejected with a temporary
error. Spammers often will not try again later,
even though it is required per RFC.
