@delta

>Any sufficiently advanced P2P messenger will eventually re-invent a custom, partial form of e-mail

That's correct, pure P2P is a pipe dream. The solution, however, is federation, and email is only one protocol among many.

ActivityPub should work just as well, but it is modern, extensible, with a rapidly growing userbase, and may eventually replace email. Consider supporting it. The secure fediverse messenger still doesn't exist, and many have promised to build one, but you can be the first who actually delivers.