I think the market is:

    1. anyone who wants a clean inbox -- maybe less profitable
    2. any company that wants better protection against phishing attacks -- maybe more profitable
    3. Edit to add: someone pointed to https://www.mailinblack.com/ I don't speak French, but that's apparently a company that (somehow) provides a service similar to the description here to high-profile targets.

If the skateboard version of this seems technically do-able without having to convince the entire world to switch to a new protocol I'd be happy to jump into a google doc to noodle it further.

On the surface it seems like:

    1. it would have to rely on email addresses alone -- I don't know another way to transmit information from sender to receiver that fits within current "what's your email?" standards.
    2. it would have to have a sparse enough address space that accidental collisions are unlikely -- so maybe 8 alphanumerics for ~41 bits?

But if you have other ideas I'd love to hear them -- griping about email has been a thing of mine for 20 years...

Rather than start from scratch, build cake slices to test the theory out. Look at something like OPA, it provides a generic policy language with a pluggable framework. You could extend postfix if it is not supported and then implement the policies you described. Then look for market fit and pivot as necessary

I have nothing going on right now and looking for things to fill the void. Let’s start a google doc and build an mvp. Worst case, we have a GitHub repo with our names on it :D