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Starting today, every app you use to access your ProtonMail inbox is open source and has passed an independent security audit. To that end, open source has long been a priority at Proton. Our web app has been open source since 2015, our iOS app is open source, our desktop Bridge app is open source, and all ProtonVPN apps are open source.
This means that all Proton apps that are out of beta are open source.
Today we add the ProtonMail Android app to this list. The code is available on our GitHub page.

Source: The ProtonMail Android app is open source