Here’s a quick and dirty tip that I grabbed from Jacques Bensimon. If you have ever tried to create a MAPI connection to mailbox user that is hidden from the Global Address List, you might have been rejected using the normal alias since Outlook cannot properly find the mailbox. Next time, try the fully distinguished name : /o=IPM/ou=NY/cn=Recipients/cn=ccostanzo for example. This will allow you make the connection.
If you are not sure what the DN could be, check your own profile @
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\Outlook\13dbb0c8aa05101a9bb000aa002fc45a
From that key location, you should be able to see the syntax for your own environment and hack it together for the mailbox you are trying to access.