If someone has shared a mailbox with you but it is not showing in Outlook, these are the steps you should take to bring it up.
- Click on File
- Go to Account Settings -> Account Settings
- In the pop-out window double-click on your email address:
- Click on "More Settings"
- Click on the "Advanced" Tab, ensure the "Download Shared Folders" checkbox is unchecked and click OK
- Close and reopen Outlook
You should now see your shared mailbox appear under your emails within Outlook