Pay-each-recipient is a great use for a cryptocurrency. The incentives align senders to receivers without requiring all of our correspondence to be read by intermediate machines outside of our control to block nuisance email.
but then it makes email so environmentally unfriendly, may as well just send dead trees.
edit: interesting, didn't know the idea of arbitrarily expensive computations as adding costs to combat abuse was first presented in the early 90s: https://en.wikipedia.org/wiki/Cynthia_Dwork
I remember reading somewhere in the 90's that Bill Gates proposed this as a potential solution for spam.
If I remember correctly his proposal was to have the sender solve a cryptographic puzzle, which takes a bit of CPU time. For normal every day use you wouldn't notice, but this subtle costs and throughput limit would make spamming no longer economically viable.
Cue that email solutions checklist meme.