I'm certain Satoshi was thinking of Hashcash! But I did mean what I said, though it wasn't until Martti Malmi released his emails with Satoshi that I knew this, and so might not be well known:
https://mmalmi.github.io/satoshi/
> "[this next bit turned out to be very controversial. there is extreme
prejudice against spam solutions, especially proof-of-work.]
> It can already be used for pay-to-send e-mail. The send dialog is
resizeable and you can enter as long of a message as you like.
It's sent directly when it connects. The recipient doubleclicks
on the transaction to see the full message. If someone famous is
getting more e-mail than they can read, but would still like to
have a way for fans to contact them, they could set up Bitcoin and
give out the IP address on their website. "Send X bitcoins to my
priority hotline at this IP and I'll read the message personally."