1) Razor technology integrated (To check against known spam)      http://razor.sourceforge.net/

Vipul's Razor is a distributed, collaborative, spam detection and filtering network. Through user contribution, Razor establishes a distributed and constantly updating catalogue of spam in propagation that is consulted by email clients to filter out known spam. Vipul's Razor is a distributed, collaborative, spam detection and filtering network. Through user contribution, Razor establishes a distributed and constantly updating catalogue of spam in propagation that is consulted by email clients to filter out known spam.

2) Sender Permitted From (SPF) support  http://spf.pobox.com/

With these two functions, Spamfilter ISP could be almost 100% effective at removing spam along with its current rbl/blacklist/Bayesian filtering.