Any email. But the receiver could refund/credit/whitelist certain senders; there's no reason this mechanism would have outsized impact on hobby lists other than the subscriber would have to somehow mark them as "safe".

In theory, this whole approach should work pretty well. I doubt the practical hurdles are surmountable, however.

The "prove-work-to-deliver" approach seems to have all the same benefits without a complicated, slow, and privacy-impacting ledger.