No, EMail has fundamentally bad UX for a lot of use case slack and similar are used for.
> problem IMO is how to replace SMTP.
Sadly SMTP is probably one of the parts of Mail which have aged best. Enforcing the usage of some (currently by design optional) features wrt. authentication and similar at the cost of backwards compatibility and you have all you need from the delivery protocol.
BUT:
- IMAP and similar is much worse.
- Mail bodies are a big mess it's always fascinating for me that mail interoperability works at all in practice (again you can clean it up a lot, theoretically, but backwards compatibility would be gone).
- DMARC, DKIM and SPIF which handle mail authenticity have a lot of rough corners and again for backward compatibility are optional. Again it's not to hard to improve on but would brake backwards compatibility.
The main reason mail still matters is because it's backwards compatibility, not just with older software but also with new software still using old patterns because of the (relative to the gain) insane amount of work you need to put into all kinds of mail related components. But then exactly that backwards compatibility is what.
(Yes, I have read the "Why TMTP?" link and I have written software for many parts around mail including SMTP, and mail encoding. The idea that SMTP is at the root of the problem seems to me very strange. Especially given that like I mentioned literally every other part of mail is worse then SMTP by multiple degrees...)
EDIT: Just to prevent misunderstandings one core feature of mail is the separation of mail delivery and mail authenticity, in the sense that you don't need the mailman to prove the authenticity of a mail. At most the legal/correct/authentic delivery.