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.