Hashcash

Hashcash is a form of proof-of-work system used to limit email spam and denial-of-service attacks. In simplest terms, it requires computers that send messages to do a certain amount of computational work before being able to complete the message transmission. This means that spammers must spend time and resources on computations in order for their messages to be sent, which discourages them from sending large amounts of spam emails. The complexities of these computations are meant to make sure that only legitimate users can send messages without encountering any issues or problems.