all i can say is that i need to log into outlook to log into teams, because logging into teams directly is broken, but the cross site cookie work… Thankfully I don’t even have an MS account, but I do implore you to check out the cookies and see for yourself that they will have a specific list of domains they are valid for. Open the browser developer menu on an Outlook tab (F12), navigate to “Storage” → “Cookies” (may be named slightly differently in different browsers), and look at the “Domain” column in the table.