After several years of using Boxer, which became prone to crashing with never ending wait times in the last months, I turned to dispatch which soon proved highly superior to everything I had used before. It integrates nicely in my workflow (2Do, Pocket...), has a highly usable UI, and is very stable. A possible improvement would be to offer some integration with Sanebox.
Best mail app for mail geeks and power users, I highly recommend it.
Apicius34 about Dispatch: Email meets GTD, v3.1.2