HackThePlanet has a pointer to a paper which describes yet another possible method for combating spam: Greylisting. Greylisting is described as a combination black/white listing with automatic maintenance thrown in.
The short version is that the software keeps track of "relationships" which are made up of the sending IP, the sender, and the recipient. Oh! This software is intended to be built into an MTA (server) rather than a MUA (mail client).