Outlook is a once-great product that has been left to rot by a Microsoft with little lineage to the great company from the 90s and early 00s that created it.

Outlook debuted Cached Exchange Mode in the early 00s, popularizing "offline first" before it was known as that.

Now: The "new" Outlook can't even show folder unread counts correctly, even when fully online. It seems to only load a small subset of messages locally, only populating folders when you scroll past the point it loaded. (In classic Outlook, this was a setting—I understand loading all mail was not enabled by default—but it could be enabled. No longer.) It sometimes gets stuck where it won't show new mail until restarted. (Gmail has this bug too.) It forgets open mail windows when restarted. It forgets expanded folders in the folder pane when restarted (but only sometimes).

Microsoft removed the ability to show the mail/contacts/calendar navigation bar below the folder pane, and forced it to be shown on its own huge vertical bar, almost all of which is wasted space. For good measure, they did this in classic Outlook as well as "new" Outlook. There was massive backlash to this, and Microsoft plowed forward anyway. On Windows there is/was a registry setting to revert this (but intentionally, no user-facing setting). I have not checked on Mac.

To the sibling comment: Outlook for iOS is indeed great, probably only because it was an acquisition. It is not in Microsoft's DNA to build an app like this themselves anymore.

As an admin: Microsoft seems to redo the Office 365 admin interface every 2-3 years. It is an incomprehensible mess. I am also a Google Workspace admin for the past several years, and theirs is far better, and it's more or less stable over the long term.

Office 365 has been hacked by state actors recently.

I still like the Outlook UI and feature set better than Gmail (despite the "new" Outlook being a major regression), so I begrudgingly stay with Outlook/Exchange because I dislike it less than Google Workspace.

Fastmail does not give signs that they are a relevant company—they created JMAP and basically did nothing with it. Why not make a first-class Windows/Mac client, offline first, with powerful organizational features, like the Outlook of yore? Or at least, contribute to adding first class JMAP support to Thunderbird? This is your sole business.