Earlier quoted context omitted.
Satoshi Nakamoto's first visionfor Bitcoin was just this - pay-to-send email.
I think you're thinking of Hashcash? Not invented by Satoshi, and not pay-to-send exactly. It's proof-of-work for email. https://en.wikipedia.org/wiki/Hashcash
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."