postfix to have greylisting in 2.1-RELEASE!
It seems that postfix 2.1-RELEASE will include an amazing feature called “policy server”, which can be used to implement greylisting, a brand new and effective technique to fight against spammers.
OpenBSD has introduced greylisting functionality through pf(4), which does not work properly at this time writing (I am doing some work to solve the problem with help from Max Laier and Pyun YongHyeon). OpenBSD’s approach is more interesting - It not only does greylisting, but also eats resources at spammers' side if they are blacklisted by keeping the connection open and reply very slowly.
The generation III of my mail system will have this type of functionality.