I have been using Outlook for the longest time and have tried FairEmail a couple of times and simply deleted it. Tried sticking through it for about a month and I'll be sticking with it for a good long time now :)
What didn't you like about K-9? I used FairEmail for a while and was happy enough with it, but K-9 seemed like a huge upgrade when I finally made the switch.
For me personally, K9's design seemed very unintuitive and complicated, when I first tried it almost 10 years ago. FairEmail for me is a great compromise between usability and feature-richness.
K9 is probably completely different from a decade ago, but I guess I will wait until K9 has merged completely with Thunderbird, before I look into it again.
I used FairMail. Nice and minimalistic with focus on security and privacy. Also OpenSource. Bought premium back then I had it. (I don’t have Android anymore , that’s why I can’t use it)