Hello,how about new anti-spam measures? "Honeypot"-mail-adresses catch bulk-mail and distribute it to a peer-to-peer-network wich suppresses mails identical to those of the "honeypot"-adress.
To prevent poisoning, think about an own kind of "web-of-trust" for that. Only privileged IP-adresses or hostnames should be able to add mails to the supression-pool.
Regards, Andreas Wagner