[grishka]

Client to server protocol is unimportant, you could as well roll your own that suits your use case. I do feel like XMPP is a good server-to-server protocol for federated instant messaging though. The only other one I know is Matrix, and it takes a much more convoluted approach by adding the requirement that each "room" is fully distributed across all participating servers. While it's something that's nice to have, it's not worth the hassle, imo.