I remember reading somewhere in the 90's that Bill Gates proposed this as a potential solution for spam. If I remember correctly his proposal was to have the s

I remember reading somewhere in the 90's that Bill Gates proposed this as a potential solution for spam. If I remember correctly his proposal was to have the sender solve a cryptographic puzzle, which takes a bit of CPU time. For normal every day use you wouldn't notice, but this subtle costs and throughput limit would make spamming no longer economically viable.