Copy from reddit:
Hi everyone,
We’re happy to announce that the new Proton Mail Android app is now available in beta to all Proton supporters!
The app has been rewritten from the ground up to make it more stable and reliable. This provides a solid foundation for new features we plan to add soon. The app already includes one of the most requested features - Conversation mode, which is enabled by default.
You can join the beta program in 2 easy steps:
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Go to the Proton Mail app in the Google Play Store
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Scroll down to the “Become a beta tester” section and tap “Join now.”
Find more information here: https://proton.me/support/mail-android-beta.
You should receive an update if you’re already in the beta program. Note that it may take some time for your app to update. Please note that some minor features you may use daily are yet to be added to the new Mail Android app.
We plan to release this updated version of the Proton Mail Android app to everyone in the coming weeks.
Thank you for your support and for helping us test our apps! As always, we look forward to your feedback.
The Proton team
Apps are usually open sourced and made available as well as APK file after the app exits beta. Now it is in beta for everyone as of today.