I don't have the answers but I think the 10 years of failure to put a dent in spam have shown beyond the shadow of a doubt that Internet email is broken by design and bandaids are not going to fix this, no matter how many different bandaids are applied. It is time to re-engineer with the benefit of hindsight.
ready, fire, aim. we aren't hitting the target. the gun must be broken. couldn't be that we have lousy eyesight. or that the ground is shaking. folks... when we look at email as a complex human communication service, and we explain spam as an intrusion into the model of using that service constructively, and we can formulate reasonable models of "repair" that do not break the broad utility of service, and we find that we cannot engineer changes to the existing service to implement these human-level "repairs", THEN it will be time to consider "re-engineering" email.