With a default installation of Exchange 2007, when an Exchange 2003 user tries to open/view the shared calendar of an Exchange 2007 user they receive the error “Unable to display the folder. The Calendar folder could not be found.” This error is received because the Exchange 2007 server rejects the MAPI request from the user’s Exchange 2003 server. Adding the following registry value will allow the Exchange 2007 server to accept these requests from 2003 servers:
Value Name: Disabled MAPI Clients
Value Type: REG_SZ
Value Date: -6.4.9999
The value “-6.4.9999” will allow Exchange 2007 servers to accept connections from Exchange 2003 and all versions of Outlook.
You can read more about this setting at:
How to disable MAPI client access to a computer that is running Exchange Server
Technet: All versions of Outlook are allowed to access the server