Dear Aaron Crawford_210
Good day! Thank you for posting to the Microsoft Community. We are happy to help you!
First of all, we apologize for the inconvenience and inconvenience caused to your work! Based on your description, I have a rough idea of your problem, it's possible that the older version of Outlook may not be compatible with the new shared mailbox. However, you can try the following steps to force the shared mailbox to show up:
- Click File>choose account settings
- Select the email account that is not showing the shared mailbox and click "Change".
- Click "More Settings".
- Go to the "Advanced" tab.
- Click "Add" under "Mailboxes".
- Type in the name of the shared mailbox and click "OK".
- Click "OK" again to close the settings window.
- Open Outlook and see if the shared mailbox is now visible.
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Please also kindly check this official document to get more details Open and use a shared mailbox in Outlook - Microsoft Support
Meanwhile, if you still can't see shared mailbox in the old verion, please kindly try to use your account to login OWA(outlook web app) and theb check whether you can see shared mailbox, thanks.
Thank you in advance for your understanding. Your patience and cooperation will be greatly appreciated. I hope all the best!
Sincerely
Kerry Chen | Microsoft Community Moderator