An orphaned mailbox is a mailbox that exists in Microsoft Exchange but does not have an Active Directory user associated with it. These mailboxes will display in the Exchange System Manager with a red “X” icon. If you do not want to reconnect the orphaned mailbox with a user, you can delete it from the server.
Start Exchange System Manager.
Locate and expand the Mailbox Store where the orphaned mailbox resides. To locate the Mailbox Store, click on the plus sign beside "Administrative Groups" in the menu on the left side of the screen. Click the plus sign beside "First Administrative Group" to expand that menu, and click the plus sign beside "Servers" to expand your list of servers. Select the server that contains the mailbox you want to delete, and click on the Mailbox Store to see the list of mailboxes associated with it.
Right-click the orphaned mailbox that you want to remove and select "Purge" to permanently delete the mailbox.