Beautifully writtenHi Marcel,
It is sad, that you are discontinuing the project as there is nothing like FairMail. However I understand and support your decision. Working with Google is incredibly frustrating and you have to deal with every whim they have with no way to defend yourself. While I would love to see FairMail maintained I don't want the person behind it to feel shackled by it.
I know first hand how frustrating working with Google is. They make random decisions, do stupid API changes and more ****. In theory Android is open, but thats not really true, because if you want to go outside the Google bubble, you either have to be in China or reach only a tiny audience. For developers who do it mostly because they want to, it becomes work rather than a hobby. I maintained an open-source app myself and over time I spent more time just updating it for newer Android version than developing anything new. I ended up burnt out and basically switched to maintenance mode only.
I hope the legacy of FairMail will continue in the future in someway. Once you take some time from this, please give another thought to the future of FairMail. It's fine if your decision stays the same, but sometimes time is enough to give you another perspective and maybe find a new way forward, even if you are not the one who maintains it. You put so much work into it that I think it would be a shame just let it collect virtual dust.
I wish you the best of luck on whatever you decide to do next. Thank you for creating FairMail in the first place. It's obvious it took a lot of work and passion to create it.