“K-9 Mail is looking to raise 1,000 EUR per week (~52,000 EUR per year) to fund development.”
K-9 has a lot of users and one developer doing most of the work. The last stable release came out in 2018 and barely runs on current Android. Google keeps changing the platform rules, and every change lands on volunteers. The best open email app on Android is asking for less per year than one junior salary.