Along with seemingly everyone else, I had a quick look at my options to migrate away from Apple over the last few days. First thing I looked for was a decent mail app, was immediately put off the whole idea, exactly as you describe.
This is probably the most painful thing on desktop at the moment, because no one gives a s* to desktop mail apps these days.
I recently switched back to evolution from Thunderbird, but where one if great, the other falls short ( in this case, GPG with Evolution is _painful_ ). Geary is... well, not mature, Claws and Sylpheed are a bit too oldschool and mutt is ... well, mutt.
I'd love to see something like Rainloop[1] as a real desktop app. Mailpile[2] is a nice idea, but is nowhere even close to the usability of Rainloop.
Well Geary (and the elementary fork Pantheon Mail) are quite good for basic use. There are still a few bugs and their codebases are quite hell to work with (see [1] for my review of them). This document motivates my little side project, but, lately, it hasn't seen much direct attention as I've been working on some stuff upstream, in elementary and Evolution, for this project.
I recently switched to Mint Cinnamon and I'm completely satisfied. There is not one thing I miss from Mac OS X and many where GNU/Linux is much better.
