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Where to get ProtonMail
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F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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From what I understand, that is not totally true. Basic does target the newer Android versions, but from what I understand, they are bringing the main client up to date as well. I'm using the main client at the moment, and it used to target 25, and now it's targeting 29. It's obviously still not the newest, but it has increased quite a bit, and I think it will continue to do so.