Microsoft's AI BS has apparently targeted this copy of Win11 to force the new outlook app instead of Windows Mail. The auto forced switch, where Mail starts to open & is immediately closed as outlook starts up, happens more frequently than our other copies of Windows. Yesterday they removed the toggle switch to go back to Mail, with a note that you could still switch back in Settings. Could be wrong, could have misread it, and since it was a one-time thing, I can't double check, but there is nothing in Settings that I could find to switch back to Mail, & I've checked semi-obsessively Many times. It doesn't appear to be something in the registry, and Googling [for hours] hasn't done any good. Neither has Copilot for that matter.
I did however eventually find a tentative, & simple fix, though I don't know yet how long it will last. To go back to using Mail without the automatic switch to outlook, edit C:\ Users\ [UserName]\ AppData\ Local\ Packages\ microsoft.windowscommunicationsapps_8wekyb3d8bbwe\ LocalState\ Migration\ callback.txt, making sure it says: permaToggle":false,"allowAutoToggle":false. I don't know if it will auto-switch to outlook in the future, or if/when it does the toggle to go back to Mail will be there or not -- I suspect that's another issue &/or possibly a new version of outlook -- but it does let me open and use Mail, at least for now.