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.