The original invention of PoW, as well as the idea of using it for email, was years earlier; see Naor and Dwork's "Pricing via Processing, Or, Combatting Junk M

The original invention of PoW, as well as the idea of using it for email, was years earlier; see Naor and Dwork's "Pricing via Processing, Or, Combatting Junk Mail" in CRYPTO'92.