First Thunderbird then this, what's a native-email-app user to do? I wonder what ever happened to that Letters email app for OS X that people were wanting to build? There was a flurry of activity and complaints about Mail.app and then nothing.
https://github.com/ccgus/letters
I remember thinking at the time it sounded quite interesting, so it's a shame.
I'm guessing the need for it evaporated with the improvements in Mail.app and the development of other native clients.