Why not solve spam emails by letting senders work on some cryptographic puzzle (e.g. Proof of Work) that recipients can verify. This would prohibit spammers from sending massive amounts of email, since the required proof of work would scale linearly with the number of recipients.For example let the sender search for a hash that must satisfy following condition: hash(recipient-email-address+message-content+nonce) % 42 == 0If the hash does not match this condition, proof of work is missing and the email likely spam.Another parameter could be the amount of work done by choosing different hash functions or hash functions that have parameters that tweak their run time.