The Stamp Problem:
https://en.wikipedia.org/wiki/Postage_stamp#The_nineteenth_century
Before the introduction of postage stamps, mail in the United Kingdom was paid for by the recipient, a system that was associated with an irresolvable problem: the costs of delivering mail were not recoverable by the postal service when recipients were unable or unwilling to pay for delivered items, and senders had no incentive to restrict the number, size, or weight of items sent, whether or not they would ultimately be paid for.
email spam exists because the system pays for delivery, not the sender.
This shows up in many places in human society.
for extra bonus fun
THIS IS WHERE BITCOIN STARTED!
someone decided to make stamps for email
The latest incarnation of this idea is Anubis! https://en.wikipedia.org/wiki/Anubis_(software)
If you've seen the cat eared anime girl before a website would load, your computer has done math that is the equivalent of a postage stamp!
@xentrac that's a good point!
@shapr Anubis (and Hashcash) lacks a crucial thing that postage stamps supply: payment to the providers of the service being DoSed.