He’s thought a lot about spam. More than most.
See Hashcash:
https://en.m.wikipedia.org/wiki/Hashcash
“Hashcash is a proof-of-work system used to limit email spam and denial-of-service attacks. Hashcash was proposed in 1997 by Adam Back”
https://blossom.primal.net/6459fc94c07f6aa308ce2a9df...
