hashcash headers (proof-of-work) for spam protection looks really interesting but is not really used in the wild: https://security.stackexchange.com/a/118182/20

hashcash headers (proof-of-work) for spam protection looks really interesting but is not really used in the wild: https://security.stackexchange.com/a/118182/202065 https://security.stackexchange.com/a/118182/202065