If we reach a point where the spaminess of an email can be correctly determined based on the reputation of the domain from which it was purported to be sent (after some cryptographic and policy checks), then is the problem of spam equivalent to the problem of bootstrapping trust for new domains?

I suppose technically there is the problem that a domain with a good reputation can be hacked or bought, but the costs of that happening should be orders of magnitude higher than the cost of buying a new domain.

Requiring new mail-serving domains to post a bond for good behaviour seems like it would similarly increase the costs of buying a good reputation, but the problem is coming up with a fair process to decide when a bond should be forfeited.

I'd be reluctant to give the ITU any control over the global email network, so instead perhaps there could be some system for countries to delegate the bond-forfeiting power to companies of their choosing, with each country having a vote in proportion to the cube root of their population size. All existing domains would be grandfathered in to not need bonds, so the potential damage from abuse of this system should be minimal.