
Cryptographer Adam Back and Hashcash - Blockwhat
https://medium.com/blockwhat/97-anybody-interested-in-some-hash-cash-7fd422dc5e79
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doomrobo
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.

