If these are basic questions, pardon my limited knowledge of email-transport internals.
- May be the transport is better than HTTP, but HTTP is well understood and easy to debug for developers (debugging a web-app is easy). Similarly, a JSON payload will bring structure than the current way of creating boundaries. Moving to HTTP+JSON will allow far easier access to developers who want to build on top of it, or self-hosting. Get a domain, run a web-app, set a few records and you are done.
- Put a file in outbox and it get's out. From my outlook experience, it seems to be a cron that scans a particular DB query. Should still be possible.
- I have no idea on how onion routers etc work so won't comment on that.
- And lastly, if Google <> MS, Outlook <> Exchange use a proprietary protocol then I will read that as an indication to improve current standard.