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 :)
For Gmail users on Mac desktop, I really can’t recommend Minestream enough, if a paid app isn’t a dealbreaker. It’s designed by ex-Applers and implements gmail’s API completely— even to the point of the signature editor and vacation auto responder and having a mode that only shows unread messages and starred messages in your inbox (which is kind of a badass way to stay on top of busy mailboxes).