> I think that part actually works fairly well today (not optimally, but well enough in practice).

I haven't done much with IMAP, but I did setup a watcher for my mailboxes that uses the IDLE extension. I consider it a flaw that I need to create a separate connection, each needing their own session, for each folder I want to watch simultaneously. Servers understandably limit the amount of simultaneous connections you can have, so that limits the amount of folders you can watch simultaneously.

I don't know if JMAP fixes that, and I don't know if I'd prefer JMAP over IMAP as a protocol, but there are things that could be better with IMAP.

> The part that really needs fixing is SMTP.

What's wrong with SMTP?