It's a great idea. It has very deep roots. Heres a short history of it's role in creating Bitcoin:

In the 90's, hackers posted damning material on the Church of Scientology. To do so, they used anonymous remailers, a tool envisioned by David Chaum, and worked on by (Satoshi candidates) Finney and later Sassaman. The Church struck back by spamming the Usenet groups. An idea for stopping the spam was to charge a fee. However, any existing way of doing so revealed the senders identity, opening the poster to litigation. Adam Back (another candidate) then created HashCash, powered by Proof of Work, which was later borrowed by Satoshi.

Heres a product thats working at the moment, reacher.me, as tweeted by Jack Mallers.
(Only thing is it requires a Strike account, which is not available everywhere and KYCs.) https://twitter.com/jackmallers/status/1521253419318710272?t=2sB8HEl-_Y5-u_UlzwegvA&s=19

I also heard this idea on a podcast a bit back. I reached out to the developer and he said the project is being put on hokd, because the person who he's working with is Ukrainian and has his hands full. Heres the podcast: https://bitcointv.com/w/8yurCAmimz3VKDpbRLzdgM

Most recently, I heard someone building a whole system on Lightning, called Vida which includes the paywalled email, as well as phone calls, video calls, zoom (streaming or e-conferences) with a sats/min thing as well. Heres the founder on Kevin Rooke https://www.kevinrooke.com/podcasts/e68-lyle-pratt-on-building-vida-communicating-on-lightning-and-helping-anyone-earn-for-their-time

Anyway, would LOVE to use such a product, keep me updated!