In Microsoft Outlook, you try expand a contact group in a public folder that contains a nested contact group. However, the contact group does not expand, and the text "<Unknown>" appears at the end of the group name.
To resolve this issue, right-click the Contact folder in the Public Folder, and then click Properties. On the Outlook Address Book tab, select the Show this folder as an email address book option.
A similar issue occurs if another user adds a contact group from their own contacts to the public folder contact group, and then a different user tries to send an email message to that contact group. This scenario is not resolved by the Show this folder as an email address book option.