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 :)
I used AquaMail for years, but they just decided that this fall they will go subscription, and old paid users who bought the app will not get all features. So, I've transitioned to the paid (one time purchase) version of Nine email app(by 9folders). It is equally capable, working with my free accounts and my O365 corporate account (including synching calendar). I looked at several open source options, and none were competent to deal with my Work O365 stuff.