a thoughtful use of digital
The best android open source Firewall (+ adblocker / vpn ) I found so far is Rethink. It can have some problems (i.g., recently, with Whatsapp or Fastnews, blocking them from the Net), but you can solve them quite easily.
Not so good I tried apps such as Athena, or De1984, or InviZible Pro: all very difficult to set properly. Indeed they block either too much or to little (your apps from the Net).
Rethink, on the contrary, can let you set exactly what should be blocked and what allowed.
K9-Mail is a good app, but unfortunately rarely updated, in the last year. Besides, it has had an annoying cryptography bug and a notification bug.
So we tried FairEmail, that is definitely more accessorized, but not really privacy-friendly as they pretend, and not so open source: impossible to contact the developer, and pro functionalities are blocked until you pay something.
So, we went back to K9-Mail, that is absolutely open-source, even more “spartan”, but in its latest release without cryptography bug and notification bug.