[wongarsu]

That's where we got the idea of proof-of-work in the 90s [1]. The problem is that this makes legitimate use cases like mailing lists very expensive, while providing little protection against spam from botnets.

[1] https://en.wikipedia.org/wiki/Hashcash

[rightbyte]

Obviously a subscriber to a mailinglist need to set the cost to 0 for the list sender.