Its a message definition and protocol, addresses look like @user@domain, anyone can run a host, and threaded messages are linked by cryptographic parent hashes..
The idea is to take the best from email: open protocol, domain ownership, interoperability (unsolicited mail is a feature not a bug), and the best from closed instant messaging rebuilt: efficient binary messages, conversational threads, sender verification, message integrity etc. built-in. Originally envisaged for human-to-human messaging but partculalrly interesting time right now with human-to-agent and agent-to-agent messaging...
The OSS stack is up and running: Go host, Dockerised full setup, CLI, Web API, and a spec nearing v1.0. Did Show HN post week ago: https://markmnl.github.io/fmsg/show-hn.html
Seeking feedback, criticism, validation :) protocol bikeshedding, and especially interest from founding-engineer types who want to help build an open messaging ecosystem rather than another closed app..