tyiu on Nostr: Protonmail is open-sourced under GPLv3. ...

Protonmail is open-sourced under GPLv3.
https://github.com/ProtonMail/proton-mail-android/blob/release/LICENSE

I’m using Obtainium instead of F-Droid so that I can choose to install and auto-update from the developer’s source website APKs in addition to centralized repositories like F-Droid.