This was the concept behind HashCash, the concept behind BitCoin/Cryptocurrencies in general. The idea being that each email would do a non-trivial amount of w

This was the concept behind HashCash, the concept behind BitCoin/Cryptocurrencies in general. The idea being that each email would do a non-trivial amount of work, such compute a valid hash - encode it in the message header. This message header can be checked quickly on receivers side, but takes time for the sender. This makes sending a large amount of email quickly infeasible without a significant amount of compute power. https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash