Proof-of-work was originally proposed precisely for email (specifically spam), by Dwork and Naor back in 1993! https://en.wikipedia.org/wiki/Proof_of_work http

Proof-of-work was originally proposed precisely for email (specifically spam), by Dwork and Naor back in 1993! https://en.wikipedia.org/wiki/Proof_of_work https://en.wikipedia.org/wiki/Proof_of_work