From the creator of Nylas Mail comes Mailspring. You can run it on Mac, Windows, and Linux (it’s an Electron app, so your feelings about trying it out will depend upon your feelings about Electron apps). The free version is enough for just about anyone, although there’s a pro version that’s kinda pricey. It runs better on Mac than Windows, due to an issue with its very own white screen of death. I can confirm that it’s maddening and it gives me the willies that it’s been documented and unfixed for as long as it has. I really like it, with super-fast keyboard shortcuts and an information-dense display.