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Which are your most useful apps?
(lemmy.ml)
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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Fair Mail works way better for me than K9
I find FairMail better suited for my needs, too, but both are great e-mail clients with extensible customizability. It is up to your preferences, I suppose.