Overview
When an account is provided "FullAccess" permissions directly to another account the shared mailbox will automatically show up in the left side folder list in Outlook desktop.
Sometimes it will require adding the account manually to be able to see it in the folder list.
Click on "File" from the top left.
Select "Accounts Settings" and "Account Settings..."
Click on your account and select "Change...". The dialog can also be opened by double-clicking on your account.
In the "Exchange Account Settings" dialog, click on "More Settings".
Click on the "Advanced" tab and then select "Add..."
In the "Add Mailbox" textbox enter the other mailbox and click on "OK".
The account will be added to the "Mailboxes" list. Click on "OK" to close the dialog.
Select "Next" in "Exchange Account Settings".
Click on "Done".
Click on "Close".
Restart Outlook.
If you are using Outlook’s new view, please follow these steps to add a shared mailbox.
Right-click on your mailbox and select “add shared folder or mailbox”
Next, enter the full email address of the shared mailbox and click add.
Once you have added the shared mailbox scroll down the folder structure of Outlook on the left-hand side until you find “Shared with me”. You will need to click on the carrot to the left to expand the folder and see the shared mailbox within.
Note: It will take some time to download emails for the shared mailbox. If you are adding the account for the first time it will show up on the left window under your main account. You may have to click on the small triangle to expand it.