I didn't know of this one, that looks really nice! Do you know why they chose to start over when K9 already existed for a long time (it says copyright 2018-2021 on the website), or is it a fork?
FairEmail was made from scratch, and has modern (from the ground up) sync logic, that works well with modern battery saving measures in Android. Push mail by default, and low power consumption during sync. I believe it actually works better than Gmail (which has the advantage of Google's push infrastructure) - FairEmail just uses IMAP idle (where possible).
K9 originated in the old android AOSP email app, and I think the developer of FE wanted to cut loose from that and build it according to newer design patterns.
K9 originated in the old android AOSP email app, and I think the developer of FE wanted to cut loose from that and build it according to newer design patterns.