||Defending Your Site Against Spam, Part 2|
If you want to add in some additional protection for unlisted servers, you could add in Gray Listing which will buy you an hour of spam shielding before a new IP can harass you.
Check out http://projects.puremagic.com/greylisting/
Gray Listing for the first hour to detect an attack, RBL to keep them at bay after that hour. Perhaps trusted servers could bypass Gray Listing, and just use Gray Listing for unknown IPs.