Client Access Server Planning http://technet.microsoft.com/en-us/library/bb125239.aspx
When planning your Exchange 2010 deployment, you must have at least one computer with the Client Access server role installed in every Active Directory site that contains Exchange 2010 mailboxes. You can have multiple computers with the Client Access server role installed within each Active Directory site. To provide external client access, at least one Client Access server within your organization must be Internet-facing.
For more information about namespace planning and Client Access servers, see Understanding Client Access Server Namespaces.
