Using New Outlook on Windows for personal email, calendar, and contact management
I gave up on Outlook and this Unified Inbox problem. MS is unable to fix this. The easy solution is to install BetterBird from betterbird dot eu - this is an optimised version of Thunderbird. It also solves some issues of Thunderbird with the authentication issues since the changes from dear MS. I have 20 email accounts from different providers and it has been working without problems for 6 months.