I’ve been put onto a great little tool called policyd developed by Cami Sardinha. It provides for white, black and grey listing, as well as rate limiting and blocking of users and MTAs. From Cami himself: Policyd is an anti-spam plugin for Postfix (written in C) that does Greylisting, Sender-(envelope, SASL or host / ip)-based throttling (on messages and/or volume per defined time unit), Recipient rate limiting, Spamtrap monitoring / blacklisting, HELO auto blacklisting and HELO randomization preventation.