To be honest, I’m not sure why end-users would want JMAP for e-mail access.
It would be interesting if they do successfully roll out all of these additional RFC proposals providing a cohesive “groupware” protocol covering calendering, contacts, file shares, etc, we see notable server implementations, and interest is enough to drive client support.
That’s a lot of “ifs”.
People say things like that, and I wonder if I’ve just been living in a gilded tower of using Apple Mail with decent IMAP server implementations.
I’m also pretty familiar with the wire protocol and its implementation — it’s never struck me as particularly horrible.
A new protocol isn’t likely to solve the problem of poorly implemented clients and servers — e.g. Google doesn’t really care about good IMAP support, so they’re unlikely to care much about JMAP, either. They just want you to use their webapp.
Shameless plug for a client with true offline-first IMAP support:
Mail.app is what NeXT used internally, and Apple uses to this day AFAIK. Steve Jobs historically paid a lot of attention to it and wasn’t shy about weighing in on any changes.
Most of the complaints that I’ve heard about it seemed to stem from poor IMAP servers (e.g. Gmail), but it sounds like your knowledge in the space would be a lot more detailed and recent than mine, so I would be very interested in your thoughts.
I've written about my experience and motivations here:
https://marcoapp.io/blog/marco-an-introduction
Gmail does indeed _intentionally_ provide poor IMAP service. But the long and short of it is that Apple Mail simply isn't a first-class product. It's an afterthought.