Coding it up isn't the problem; network effects are.

Simply using such an app is useless to prevent incoming spam. You've got to convince the Spammers to use that system to get the benefits you're looking for. That takes nothing short of convincing the majority of the world to start paying for their email and ditch free email first.

Good luck with that.

21.co and earn.com actually tried to solve this issue, however coinbase seems to have acqhired or did a strategic buyout of them.

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/

I was talking with André Neves at @zbd a while ago, and he mentioned this exact use case is on the ZEBEDEE product roadmap.

Or simply, if you host your own server and you know how to manage it, create a blacklist based on senders, IP, domain, subject or whatever you want and create a rule for RFC 550 reject message saying: "If you want to send me an unsolicited message, please pay to this LNRUL or LN address x amount of sats, including your email address."

Or if you have that contact on a webpage, put a paywall that will force them to pay first before filling the contact form. https://github.com/cryptoteun/awesome-lnbits

To make something like this happen, as a first step one would have to create a somewhat social network of people that are used to paying sats to each other for content and value provided... ;)

You used to be able to do it with earn.com but I haven't seen any replacement for it since coinbase bought it and I assume shut it down

See also:

Jack Mallers announces a solution to spam? #26458 https://nitter.it/jackmallers/status/1521253419318710272

and

How To Curb Email Spam With Bitcoin #26458 https://thebitcoinmanual.com/articles/curb-email-spam-btc

Unless you can get some sort of anti-spam payment protocol adopted as part of the standard email protocols servers use now, it's a lost cause.

But fortunately much of our communication is moving to messaging apps and it's not too late for something open source like Matrix to adopt an antispam mechanism based on Lightning.

aka hash cash