Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi
Thanks for your feedback!
Based on your description, first of all, I need to confirm how do you add a shared mailbox in Outlook, through automatic mapping or manual addition? If it is added automatically, please disable the following option and check it. Please kindly refer to the screenshot for details:
If your problem persists, to better help you and not waste more time, please kindly try to create a service ticket in the Microsoft 365 admin center to support > service requests through navigation, as they have more access permissions than on this forum, and professional engineers will further help you.
Note: if you are not using admin account, please kindly ask admin within your organization for help.
Wish you have a wonderful day and always stay safe!
Sincerely
Kerry Chen | Microsoft Community Moderator