For some particular issues, if you don’t have the means to create a new System Attendant Mailbox (that is by moving all users mailboxes from the store where the SA Mailbox resides)
We need the following tool: http://mfcmapi.codeplex.com/
1.) Create an Outlook Profile, in Online Mode.
2.) Launch MfcMapi.exe, select Session -> Logon Only ( Does Not display stores ), choose the profile created under the previous step, select MDB – get Mailbox Table.
3.) Enter the name of the server that owns the store/mailbox you wish to view.
4.) Double click the SystemMailbox.
To reset the FreeBusy messages if FreeBusy information are missing when some users are booking meetings, search for the system messages for Free/Busy informations :
5.) Under SpecialPrivateFolderFreeBusyStorage , you will see the Free Busy Messages, select them, right click them and do a hard delete and you're done.
From: Patrick from his blog - http://blogs.technet.com/b/ehlro/archive/2010/11/01/using-mfcmapi-to-connect-to-the-exchange-system-attendant-mailbox.aspx