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.

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

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

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