Each mailserver could keep a cryptographically verified list, the list is distributed via some P2P mechanism, and DoS directed at the 'source' of the service only interrupts updates, and only does so until the source slips an updated copy of the list to a few peers, and then the update spreads. Spam is an economic activity and they won't DoS a source if they know it won't help their situation.
If anyone who attempts to distribute such a list is DoSed to oblivion, people will stop being willing to distribute such a list. Yes, spam is an economic activity, but spammers may engage in long-term planning. You can't keep the list of distributors secret. I'd be very interested in techiques that overcome this problem. I've been looking into tricking existing widely-deployed infrastructures into acting a distributors, but this raises both ethical and technical questions. DS