End-to-end crypto solutions on the client side, such as S/MIME & PGP have existed for nearly 20 years.
But for Android users, there is simply no decent e-mail app in which supports this type of required security in Google Play store, while also supporting office365 (required for work), tablet mode, and threaded message viewing.
Stock mail app, Gmail, Outlook, Touchdown, Nine, etc., none of these apps meet of these criteria. And don't mention Samsung Knox, which is only available with stock Samsung ROM on its hardware, and won't install or work with custom ROM's on its hardware such as cyanogenmod.
I very much prefer Android over iOS, but wished there was at least one decent and secure android mail app which meet my criteria, the way that iOS stock mail app does. Not to mention having the extremely handy in-app file attachment preview of pdf, word, powerpoint, excel, etc. which iOS stock mail app provides.
If any decent Android mail app ever does go on sale, I would be happy to pay up to $100 for it, especially for something close to iOS stock mail app. Since this would be a bargain compared to switching back to iOS just for decent mail.