Ugh. Yet another intriguing email startup being acquihired and killed off by a more-established tech company (see also: Sparrow). It's 2015 and I still bounce around email clients every couple months because all of the major options have substantial flaws.
What do you not like about Outlook or Gmail (I think the market leaders)?
Outlook (incidentally, another example of my original complaint, as it's just former email startup Acompli with a fresh skin although obviously it's probably not getting shut down anytime too soon) doesn't have very good message searching for Gmail, which is a common complaint I have for non-Google mail apps. Obviously, I get that Google has a huge built-in advantage here; but if your email app is going to support the "Mailbox paradigm" of abandoning folders/traditional labels in favor of just marking things as done, it also needs to have search functionality that can back it up. Outlook was also really buggy for me when I last tried it, although that was a little while ago so it may have improved.