K9 Mail was my goto email client for many years. I eventually decided to stop waiting for them to implement OAUTH2 -- it clearly was something that wasn't high on their list to actually get done... So about 2 years ago I then moved to Fairmail, another open source app, under very active development with a very responsive developer, with full OAUTH2 support. I'm been quite happy with it.
I see that K9 got OAUTH2 support in the last couple of weeks. Good for them! I'm happy that, in the future, K9 Mail will be a good alternative if I ever decide I'm unhappy with Fairmail.
