Fun fact: Bitcoin's proof-of-work algorithm is based on Hashcash, which was an anti-spam measure along those lines: a message contains a partial hash brute-forc

Fun fact: Bitcoin's proof-of-work algorithm is based on Hashcash, which was an anti-spam measure along those lines: a message contains a partial hash brute-force that takes the sender about a second of CPU time to compute (and depends on the recipient). But it didn't take off, and is problematic when spammers have vast amounts of CPU resources in the form of botnets.