[maxnoe]

I don't see why this should not be possible.

Isn't this mainly about adding a a subject metadata field an a client that just displays messages differently, enables sorting into directories and so on?

Is there a real technical difference between messaging and long form emails that I don't see?

[ForHackernews]

Email is based on open, federated protocols. Every successful instant messaging service (sorry XMPP) as been a single closed provider.