For those of you who are new to Exchange 2007 administration, you need to be aware of a significant design change when it comes to disabling and removing Exchange mailboxes.
In Exchange 2003, if you needed to disconnect a mailbox from a recipient object you would delete the mailbox in Exchange System Manager. It would not be deleted permanently until the deleted mailbox retention period for the mailbox store had been reached (30 days by default).
This is different in Exchange 2007, because the Exchange management tools have been re-designed to administer Exchange objects in AD directly. Now when you remove a mailbox, you are removing the recipient object from Active Directory and marking its mailbox for deletion.