Postgrey

Postgrey is a Postfix policy server that implements greylisting. When a mail delivery request is received, it builds a triplet of client IP, sender, and recipient. If the triplet is new or was first seen less than 5 minutes ago, the mail is temporarily rejected. Spammers and viruses typically do not retry, while legitimate mail servers do. Additional features include safe database locking, automatic cleanup of old entries, per-client and per-recipient whitelists, subnet-based recognition, and auto-whitelisting of clients that repeatedly pass the greylist.