For some reason I have it in the back of my mind that they were at one point accused of being a honeypot for US intelligence because of their association with MIT. Probably complete BS, but maybe not. Are they as open source as they claim to be? Looks like they’re on github. F-Droid seems to think they have some Google libraries or whatever that they use.
ProtonMail users, how do you like/dislike it?
General rule of thumb:
ProtoMail, touts itself as a “secure web app”, which is a contradiction.
If you use an open source app to access ProtonMail’s service, the security lies in whatever app you use. At that point, might as well send E2E encrypted mail via GMail.
TL;DR: the way most people use it, is just security theatre.