Each user must be assigned a virtual root to access Global Mailbox.
The virtual root is the first level of the directory path for a user when they are navigating the mailbox navigation pane.
To support limited visibility into the mailbox hierarchy, mailboxes are visible to the user as a relative path, while administrators see the mailbox in an absolute or full path. This concept is referred to as the virtual root .
An administrator user always has a virtual root of / (slash). If a standard user is changed to an admin user, that user is assigned a virtual root value of / (slash). No default virtual root is assigned to users. An admin must assign a virtual root before users can access Global Mailbox.