[dntrkv]

Nylas N1, Airmail, Canary, or Polymail. Take your pick. Personally, I use Nylas since it's open source and the extensibility of it is awesome.

[seanp2k2]

And because it's open-source, it won't just get shut down. Worst-case it gets abandoned or forked or suffers feature bloat (but since it's already a mail client...) or goes the way of Thunderbird.

[AlexandrB]

Thanks for the suggestions! I've tried Airmail but I found it quite sluggish. The others a new to me.

[mattryanharris]

I really can't recommend Canary enough, has everything I need. Just need them to make an iOS version.

[nkohari]

+1 for N1. Great software that keeps getting better.