As you wrote, Hashcash never saw widespread adoption, sadly. The big players all came up with their half-assed "alternate solutions" and what I saw was that spa

As you wrote, Hashcash never saw widespread adoption, sadly. The big players all came up with their half-assed "alternate solutions" and what I saw was that spammers where the first to adopt DKIM etc. I still use greylisting which keeps a remarkable number of spammers away, because they do not invest the resources to wait for n minutes for a retry. And I refuse email from clients without a proper reverse DNS entry, which more often than not appear to be hacked machines, or certain "organisations" in countries far away ...