After adding an additional mailbox in your Outlook through Account Settings, sometimes the system won’t allow you to remove it easily. Below are the steps in removing the additional mailbox from your Outlook via ADSIEdit.
1. Open ADSIEdit.msc
2. Connect to one of your primary Domain Controllers
3. Drill down to the account, or object you wish to be removed from
4. Right-click on the object and select Properties
5. Under the Attribute Editor tab, scroll down to “MSExchDelegateListLink” and remove your self from that list
6. Restart your Outlook and the additional mailbox should no longer show