[Android Apps and Games] PingFlamingo:
Right, re-distributing your app with the Pro features enabled doesn't violate any license, copyright or whatever, it's perfectly legal and I don't think you can even DCMA it as it doesn't violate anything, even if you don't allow it (you not allowing it doesn't have any legal weight).

It doesn't take much to maintain or support - all you do is get the latest FairMail code and apply the Pro feature check patch on top of it. At most you would have to keep the patch updated and maybe adapt to the build system changes once in a while. From there you can publish it to Play Store as "Joe's Mail", or, if "FairEmail" is not trademarked, you could even publish it as "FairEmail Pro (free)" along with FairMail's logo as the app icon. The person doing so won't gain anything, but they would certainly hurt your income from the app. Or if they wanted to make some money they could insert ads.

Anyway, that's just something I have been thinking about lately - that it's hard to enforce Pro features in a GPL app, any tech savvy person could enable them. It seems to be hard to monetize a GPL mobile app in general.

Technically you could make the Pro features be a downloadable add-on, so that the base app has none of Pro features code even if you decompile it, and obviously remove the Pro features' code from the GitHub repo too. The add-on would have to be GPL licensed, so you would have to give people the source code once they prove to have purchased Pro features. I think that should satisfy the GPL, and could be fully automated. This moves the goalpost of someone who wants to make a clone of your app with Pro features enabled a bit further than it is now, as they would need to have paid for the Pro features once, or get someone who has paid for it to cooperate. You could go even further and make Pro features subscription-like, i.e. charge money for every new version of Pro features and provide the source code only for the version that the user has paid for - that way the clone owner would need to keep purchasing the add-on to have the latest Pro features. Basically the grsecurity model. This would make the Pro users unhappy though, as it's pay once kind of a deal right now.

If the app was, for example, MIT, then you would have a lot easier time enforcing the Pro features being paid as you wouldn't have to provide the code for the Pro features anymore. You could still keep the source code of the base app publicly available on GitHub, well you kind of want to do that anyway if you claim your app to respect user privacy, so that people could review it and build themselves - it's hard to trust closed source apps. But then you open yourself to a small team of people forking your app and over-pacing you in its development and maintenance, and since it's MIT they can keep all their code closed. So instead you are stuck with GPL that makes sure the app remains open source regardless if a fork succeeds you, but you are left with a rather vulnerable way to monetize.

Anyway, thanks for the great app! Will be migrating to it from k-9. Might even get the Pro version if the base one works well for me.

Too bad the app had to be pulled from African and (Eur)Asian countries on AppStore just because some people can't be bothered to read the app description and see that UI is simple by design and that the paid features are optional.