I wanted to build something similar that works nicely across different devices (mac/linux) that is fast and supports keyboard-first navigation as I'm also struggling with a similar problem.

When surveying potential users I discovered that most are happy with the web gmail interface and don't really care. It sounds like a very niche requirement and it'll be very challenging to find enough customers (1k+) who would care about using a very nice email client.

Probably alpine or mutt. Both are console based and most linux systems have at least one of them installed usually.

What kind of shortcuts you're looking for? Spark has a lot (almost all primary functions has one) of keyboard shortcuts.

Mutt. The learning curve is substantial and the omnipresence of HTML and graphics is email will make the experience poor… but if you take part in multiple mailing lists and are 99% text and little else, it’s worth the hassle.

Sorry to be that guy. But Emacs ? Free forever and great at keyboard shortcuts.

Probably still needs a good text editor though.