Since Game of Thrones seems to revel in jerking us around and House of Cards is now disturbingly close to believable, USA's cyberpunk drama Mr. Robot is Android Police's pick for cable TV binge-watching. While it's not so deep in its own hacker lore that it's incomprehensible to the layman, it's surprisingly accurate in its realistic and often low-tech methods of showing hacking and counter-hacking techniques. One of those techniques is using ProtonMail, an encrypted email service that makes FBI analysts shake their fists like cartoon villains.
Considering that ProtonMail claims over a million users for its secure email system, it's kind of amazing that it took them this long to create an Android mail client. The app uses a fairly standard UI and feature set, and its end-to-end encryption means that only you and the people you're talking to have access to your respective messages. The service is free (though they would appreciate a donation), and the app allegedly works on Android 4.0.3 or later.
Except that for some reason, it's showing as incompatible with every device we've got (and between everyone here at Android Police, that's a lot of devices). This usually happens when there's a geographical lockout, perhaps to Proton's home country of Switzerland, or when developers make some mistake with their app listing. No problem: someone who managed to download the app uploaded it to APK Mirror, so users can download it to whatever device they want wherever they are. Access on the web, and the pre-existing iOS app, is also free.