Using Classic Outlook on Windows for personal email, calendar, and contact management
I have exact the same situation.
I use a M365 Business account as primary account and have secondary account which has a Basic subscription.
For some reason I have chosen for this more expensive setup (2 subscriptions, 1 Business and 1 Basic).
So, when moving to New Outlook I see all mailboxes, but can only open the M365 Business account.
To me this does not make sense at all. If this is new Microsoft policy I will need to migrate the 2 second account to a Shared Mailbox.
In your reply "... or your organization IT admins has restricted the use of multiple account in Outlook for Windows due to licensing restrictions.". Where can I find that limitation?