Proof of work is basically equivalent to proof of money, but perhaps such a system would be reasonable with a few adjustments. Firstly, the financial burden sh

Proof of work is basically equivalent to proof of money, but perhaps such a system would be reasonable with a few adjustments. Firstly, the financial burden should be on the owner of the SMTP server / domain, rather than on the users themselves. Of course, the owner would pass the costs onto their users, but the system doesn't have to require an ongoing fee, just a bond for good behaviour. (This could be done with cryptocurrency, a smart contract, and a "proof of burn" type of transaction). Secondly, and perhaps most importantly to encourage adoption, all existing registered domains should be grandfathered in, so they wouldn't have to put up a bond, whereas any future competitors would. Finally, and this is the controversial bit, there would have to be some set of entities who were able to determine whether a domain had acted in a way that should forfeit their bond (and, optionally, that a domain had acted in a positive way long enough that the bond should be refunded). In practice, though, such a cabal already exists, which is the Big Three email providers (Gmail, Outlook, Yahoo Mail). Together they already have the power to destroy any new email provider, so we might as well formalise that by giving them the power to burn these good behaviour bonds.