You can't expand a nested contact group and "Unknown" is displayed in the group name in Outlook

Applies to: Outlook 2016Outlook 2013Outlook 2010 More

Symptoms


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.

Nested Contact Group

Resolution


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.

More Information


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.