This method for manually add the shared mail box.
1. Open Outlook and Click File tab in the ribbon.
2. Click Account Settings
3. Click Account Settings from the menu
4. Select Email tab and make sure the correct account is highlighted, then click Change
5. Click More Settings
6. Click Advanced tab and click Add
7. Type the shared email Address and click OK > APPLY > OK
8. Close and restart the Outlook, shared mailbox should appear.