I'm curious: why didn't proof-of-work get adopted into some email protocol? It seems like a reasonable solution to the junk mail problem. Things like mailing li

I'm curious: why didn't proof-of-work get adopted into some email protocol? It seems like a reasonable solution to the junk mail problem. Things like mailing lists would be disadvantaged in this case, but you could reasonably disable the PoW requirement for lists you subscribe to.