In my Exchange environments, it's pretty common for me to give one user access to another user's mailbox for any number of reasons. Sometimes, the user has left the orgaization. If Joe has left, I'll give Bob access to Joe's mailbox in Exchange Management Console (giving Bob full access to Joe's mailbox) and then Bob will follow these steps to add Joe's mailbox:
Sometimes, I use this feature for common mailboxes like a email@example.com mailbox or and firstname.lastname@example.org mailbox.
Recently (the last year or so as of 5/12/14), I have found that even when you remove the mailbox using from account settings (from the same method you use to add the mailbox in Outlook), the mailbox does not disappear. An Exchange attribute needs to be changes to remove the user's mailbox.
Let's take the example of Bob opening Joe's mailbox.
Open ADSI Edit.
Drill down until you find the entry representing Joe's account.
Right click on Joe and choose Properties.
Scroll down to MSExchDelegateListLink and double click on that field
Remove Bob's account from the list.
After the steps above are done, Joe's mailbox will disappear from Bob's Outlook.
Screen shot of what you're looking for: