It is indeed amazing how many protocols we can spin up to address the same underlying problem, time and time again...  

If anyone can anonymously join the mail-sending club and send some email [until bad reputation precludes such], and achieving bad reputation results has no real-world implications, and a new network persona (e.g. domain name) is always available, then the problem could be considered intractable by initial conditions – and no amount of anti-spam protocols (no matter how brilliantly designed and engineered) should be expected to durably address the problem. 

(It might, however, be interesting to do a regression analysis on the spam mitigation protocol introduction dates – it’d be interesting to know if the expected number protocols that will need proper setup in 10, 20, 40 years…!)