Manually Removing Monitoring Mailboxes - Delete Sub Tree?

http://letsexchange.blogspot.ca/2013/08/error-deleting-database-failed-to.html

When I attempt the resolution (deleting the monitoring mailbox from AD manually) I receive a prompt

"If you select delete subtree server control check box all objects within  the subtree including all delete protected objects will be deleted and the deletion cannot be cancelled"

How can a user object contain a sub tree? and in this case dealing with a monitoring mailbox user is this the correct option to be selected?