K-9 Mail is looking to raise 1,000 EUR per week (~52,000 EUR per year) to fund development. The app has a lot of users. If everyone who is able to do so donated a small amount we should be able to reach that goal in no time.
https://k9mail.app/2021/02/14/K-9-Mail-is-looking-for-funding
https://k9mail.app/2021/02/14/K-9-Mail-is-looking-for-funding