On Windows I've used Calypso email client which was pretty awesome, but now it's pretty hard to even find a screenshot of it:

https://images.sftcdn.net/images/t_app-cover-l,f_auto/p/b57c...

Also there is a really good e-mail client The Bat, but not sure how it stands for today's HTML-first e-mail world. (I'm not using Windows anymore, and for corporate it's unfortunately Outlook, not my choice)