Fairemail is currently the top for foss email on Android. K9 may take that spot now that there's someone actively working on it with goals in mind. Thunderbird is a really solid client, so if they put similar time into k9 (but please, leave the icon lol), it may end up blowing everything else away, even the non-FOSS options.
K-9 seems promising, it is acquired by thunderbird and planning to become thunderbird bird mobile. It has office 365 support, but IIRC it is pretty convoluted to add a office 365 account. https://www.f-droid.org/en/packages/com.fsck.k9/
I used to use K9 long time ago, but had issues with notifications not working properly. I've been using something not FOSS which was OK but I wasn't 100% happy with. After reading about FairEmail on another thread the other day, I've now adopted that and so far it is working great. It has way too many options though!
Thank you for the recommendation, just downloaded it and this looks amazing!
It's incredible how long it has taken me to find something with customizable notification actions and sufficient options! This is the first time I've ever been able to have new email notifications with both Archive and Spam options.