With the desire to enable only calendar sharing from our Exchange 2007 environment to another organization's Exchange 2007 environment, we are in the process of setting up federation through the MS Federation Gateway. We've deployed a 2010 CAS server that sits behind a Forefront TMG 2010 server that's in the DMZ. We are finalizing the steps required for configuration when we came across the following: "Active Directory synchronization Active Directory synchronization must be configured for all users that need to share free/busy information between the organizations. You can either configure the Active Directory synchronization manually or use an automated Active Directory synchronization service. Learn about Active Directory synchronization at Forefront Identity Management." - http://technet.microsoft.com/en-us/library/hh310374.aspx Can anyone confirm if this is true? We don't understand why this would be required as from how we understand federation works, if Exchange 2007 is unable to locate free busy information for a remote user locally it forwards the request to the 2010 CAS server which recognizes it's for a remote domain that it has an org relationship with and sends it to the remote organization through the gateway. Why would a list of all AD users for the remote org be required to be present locally to get free busy information? We are unable to find further information online about the specific requirements for setting up and configuring DirSync just for calendar sharing only. Are we required to setup an Office365 account? Is this free? Can we use the same TMG server for DirSync as we're using for federation through the 2010 CAS? Any extra ports need to be opened? Are there any online resources available that can assist with this?