Wow - I can't believe this, on the heels of starting this thread...! - I got 2 calls this morning from different clients that their New Outlook email would not open. When they would click on the mail icon to open it, it had resorted back to the original Windows mail, and only one account was showing out of their multiple accounts.
When I would click to "try the new email client," upper right corner toggle, the program would crash and close.
Win 11 pc's, checked on a couple things, and in the end I did a system restore to Oct. 30. and it started working again. In this case, seems like a Windows update broke something on both systems.