Here's a simple mechanism which has not yet been tried seriously. Email server peering. This means that an SMTP server operator only accepts incoming mail from operators with whom they have a bilateral email peering agreement.
This has been tried in the X.400 world. I wouldn't exactly say it worked well - and I, for one, have no desire to return to X.400 style email peering.
Bilateral agreements have been shown to scale quite well whether you look at BGP peering or the world of business contracts. In any case, the fundamental need here is that for somebody to notify the email administrator that is sending spam and for that administrator to act immediately to cut the flow.
The number of agreements needed in the email world is significantly higher than what is needed for BGP. Steinar Haug, Nethelp consulting, sthaug@nethelp.no