Policyd is an anti-spam plugin for Postfix (written in C) that does greylisting, sender (envelope or SASL) based throttling (on messages and/or volume per defined time unit) and Spamtrap monitoring / blacklisting.
另一種防 Spam 的東東 - DomainKeys
網域認證鑰匙 DomainKeys
DomainKey Implementor's Tools and Library for email servers & clients (SourceForge)