Outlook honestly was not that bad for a while, but of course Microsoft does what Microsoft does. I've been using Thunderbird for about a year now and it is very full featured coming directly from outlook.
I use Outlook on my work Mac, and am forever amazed at how hard they pushed on getting me to switch to "New" Outlook, but how many features they never bothered to port over. Like, I can't export my mailbox without having to switch it back to 'old' Outlook. Calendars straight up don't work half the time and there's no obvious button to switch from a list of events for the month, back to a monthly calendar view.
Outlook for Mac is a fucking mess. I really do need to switch over to Thunderbird.
For me, Mail was a little anemic. It's nice to have a more full-featured option, but I agree that it's a mistake for MS to can the Mail App that met 90% of people's needs.
Like all open source software, there's more of a build-it-yourself ethos. I was able to customize it to my liking to replace most of the functions of Outlook. Someone here mentioned the focused View which was hit or miss to be honest, but it did a good job of filtering out most of the nonsense.
It took a little bit of time to get the settings, layout, and add-ons that I wanted for my workflow. The best thing about switching is honestly how quick it is, how easy it is to have all my emails open in one window with tabs, and above and beyond all, a super powerful, super quick search. I feel like modern searches across all software are doing away with Boolean operations, thinking they can replace it with AI rankings. A straightforward search that lets me find exactly what I'm looking for and nothing I'm not feels like a superpower in this day and age.
Looks like it uses IMAP. Nothing wrong with that. It is just common practive when locking down Exchange Online to tick the box in Conditional Access that disables "legacy protocols", which includes IMAP. I've been using eM Client which uses EWS but doesn't support push-mail so still on the look-out for something else.
I just checked and you're exactly right. It does have OAuth, but uses IMAP. In retrospect, I think I did have to talk to our sysadmin when I first set it up.