I know this has been discussed and discarded before - but I have to bring it up again. And there's a reason.
Yes, there are a number of Windows email clients. To put it simply, for one or more reasons, they all suck. Thunderbird, Postbox, EM Client, Inky, Mailbird, The Bat, Outlook, Live... Even more - AquaMail is simply the best IMAP client I've ever worked with. You got nearly everything right (I have to say "nearly"...always have to gripe about something).
No other client I've worked with combines multiple accounts, IMAP server-side searching, synchronization, presentation...
Even while at a desktop - I typically check and read my email on my Android phone. And often send from AquaMail as well - though I far prefer my full-size ergonomic keyboard & trackball to the touchscreen.
I just tried using it via Bluestacks - and it actually works! The only drawback - which I haven't devoted any particular time to - is I'm unable to select/copy/paste when composing replies. But the thought of a "pure" Windows app would be a great delight.
I don't know how difficult it would be to re-compile AquaMail, either as a standard Linux app for Linux clients, or using Cygwin for Windows clients. A full re-write makes no sense - but if just a couple of headers need to be changed to provide a desktop client - I know I'd buy it instantly. And desktop apps sell for considerably more than Android apps...