applies the mandatory properties to the mailbox
when you create a mailbox using the Exchange 2003 tools (ADUC), Your Exchange 2007 mailbox will indicate that it's a "Legacy" mailbox and will therefore be blocked from using some of the new E2k7 features
to reset the mailbox setting use this command:
Set-Mailbox -ApplyMandatoryProperties -Identity ShowInAddressBookUser -V