Microsoft recently purchased the company behind the Acompli email app, now renamed to Outlook. While it is available for Android, I have only used the iOS version. It took only a couple hours of usage before I stopped using the iOS mail app and moved all of my accounts to Acompli.
The interface is clean and it automatically organizes emails into Focused and Other lists which I have found very useful.
One of my accounts connects to Exchange 2010. I found Acompli, before the acquisition, to be more stable better performant than iOS for accessing emails, responding to event request, and calendars in general.
Website: https://www.acompli.com