@quintessence A per-email calculation cost, like some prime factoring, maybe. Configurable at the server so that as computers advance, you can require a larger computation, or if the user has allowlisted the sender, skip it.
Yes, ham email becomes more expensive, but honestly, "free" email that's paid for with ads and data harvesting has been its own kind of problem anyway.
(Also, I suspect total electricity use for all email would go DOWN with sufficient spam reduction.)