This might happen when you copy a user account in Active Directory. If the template account had a mailbox and the newly created account doesn’t have a mailbox.
Active Directory still copies the Exchange Attributes and that is why the newly created account which doesn’t have a mailbox or email address appears in the Global Address List.
To fix it : -
- Open ADSI Edit
- Look for the account and open its properties
- In the Attribute Editor find showInAddressBook
- Click Edit and remove all entries.