how about jmap.io? can I use that ?
"but e-mail !1!!!" Is probably still a number one objection from experts and power users who refuse to fathom that e-mail protocols are a viable option for instant messaging even if it demonstrably works, is fast and secure :) We'd be happy if someone engaged in a proper comparison with xmpp and matrix specs and impls, really the only three messenging protocols deployed and implemented at scale. (Can't compare that with Signal or WhatsApp which don't have wire specs!). https://github.com/deltachat/deltachat-core-rust/blob/main/standards.md
Discussion
maybe with this https://github.com/stalwartlabs/jmap-server ?