I remember something about the problem being that because your paywalled side will send an auto-response message (asking for payment), that people (especially marketers) don't like that so they mark that message from you as being spam.

So you end up tarnishing your e-mail account in doing that, not long after your normal outgoing messages start ending up being marked as spam on the recipient's end.

But there was this attempt (repo is still up, but website is defunct):

ChargedMail Monetize your email with a Lightning paywall for your Inbox https://devpost.com/software/charged-mail https://github.com/shocknet/chargedMail

And before that, a (defunct) shitcoin-related one:

BitBounce BitBounce – Email paywall that lets you get paid to receive marketing emails https://news.ycombinator.com/item?id=20475516

You can always set up a contact page on your site, with a paywall for sending. So that would be something where you aren't sharing your e-mail address and the only message that hit that from that contact page are those where the message sender has paid.

How to Create a Bitcoin/Lightning Email Paywall with BTCPay https://blog.lopp.net/how-to-create-bitcoin-email-paywall-with-btcpay/