Heya, I'm the PM for Nylas. We have a Mac mail client called Nylas Mail that's totally free that we launched on HN a couple of months ago: https://news.ycombinator.com/item?id=13417616

Built on Electron and the code is open source: https://github.com/nylas/nylas-mail

Happy to answer questions about it if you have any -

(And, yes, we are working on getting Windows and Linux out - even though it's an Electron app, there's a lot of platform specific stuff we need to do so that it doesn't go all T Rex on your memory and CPU.)