That’s an odd one. You turn off all notifications, or notifications just from outlook? Does the issue happen even if the Sonos app is closed (really doubt it), or maybe if the app is not the current focus? Does any other notifications, besides from Outlook, cause this issue?
I would guess that the issue is Outlook rather than with Sonos, as it doesn’t sound like Outlook is handling notifications properly, and somehow is sending a stop type command to Sonos. If the issue was with Sonos, then I’d expect every notification, regardless of which app sent it, to cause the issue. I have numerous notifications turned on, including a couple mail applications, and no issues.