I'm starting this because I couldn't find the Protonmail app outside the Play Store. Or, more precisely, I couldn't get it from a trusted source. This is in the context of the private phone project I'm working on. So, I used a "sacrificial" phone running stock Google Android to get the app, and then I adb pulled it as described on the private phone page.
First of all, here is this page's httpS link if you didn't get here securely already. (One day I'll do redirects and such.) The APKs will be (should be) linked securely.
I'm open to suggestions how to make this a trusted source. I would hope the context and content of this site helps, and the fact that it's been a registered domain since 2003. (Look up "whois" information.) This incarnation of the site has files going back to 2010. I can think of other things I can do, but I'm curious what feedback I get on the matter. My contact info is on my resume.
So far, I only offer Protonmail. Note that depite claims including the app itself, it DOES work without Android apps / Play Store. I can send and receive. I understand that "push" notification probably doesn't work, though. You can reload the inbox manually, though. When you run it, the app itself pops up and says that you can't run it without Play Store, but then it runs just fine.
Based on a quick look at the licenses within the file and stuff I've read, Prontonmail is free and open source (FOSS), so it is legal for me to offer it.
Below is my official "receipt." Some of the info below comes from the Protonmail Play Store entry.
Updated: February 20, 2019 Current Version: 1.10.5 Size: 22M (23263699 or 23,263,699 bytes) Offered By: ProtonMail Installs: 1,000,000+ Requires Android: 4.0.3 and up SHA256(ch.protonmail.android.v1.10.5_2019_0220.apk)= 9bde1a458fc9d6cb1207937843f04d8151dba5d0703fecf9bd34e530dd68d06b
I added the exact size and the sha256 hash. Incidentally, it appears another site out there has that hash, based on a Google Search. So, that either means we both have the real thing, or we're in collusion, possibly with Russians. Also, just because that other site advertises the sha256 doesn't mean that's really the hash of the file you get from them.
Protonmail 1.10.5 download linkor the raw directory listing