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."