How to manually add a shared mailbox in Outlook 2016 and 365
Outlook may remove shared mailboxes from your view, especially if you are frequently switching between the company network and the VPN or using Outlook without either (known as outlook anywhere).
You can manually add a shared mailbox using the method below. Note that the IT dept still needs to grant access for your account to access the shared mailbox.
These screenshots were taken with Outlook 365, other versions may vary slightly.
1. In Outlook, right click on your email address in the left pane and select Data File Properties:
2. Select Advanced:
3. Under the Advanced tab, click on Add... and then type in the email address you need to access, then click OK:
4. Click on Apply and OK to close the dialog boxes. Give Outlook some time to download the messages from the exchange server.
If you still cannot access the mailboxes then contact IT for support.