Hashcash is a proof-of-work system used to limit email spam and denial-of-service attacks. It requires the sender to perform a computational task, generating a unique code that proves they invested time and resources before sending an email. This process makes it more difficult and costly for spammers to flood inboxes with unwanted messages. Originally proposed by Adam Back in 1997, Hashcash has influenced various cryptocurrencies, including Bitcoin. In these systems, similar proof-of-work mechanisms help secure transactions and maintain the integrity of the blockchain, ensuring that participants contribute computational power to validate and record transactions.