n a Microsoft Exchange Server 2007 organization, a computer that is running Exchange 2007 that has the Client Access Server role installed can act as a proxy for other Client Access Servers within the organization. This is useful when multiple Client Access Servers (CAS) are present in different Active Directory sites in an organization and only one is exposed to the Internet.
Note: In case the Active Directory does not have multiple sites, you do not have to configure Exchange 2007 for proxying or redirection.
A Client Access Server can also perform redirection for Microsoft Office Outlook Web Access URLs. Redirection is useful when a user is connecting to a Client Access Server that is not in their local Active Directory site. Each site would have to have an Internet-facing CAS server with the ExternalURL set. Having the ExternalURL set is not a default configuration in Exchange 2007.
This topic explains how Client Access Server Proxying, Redirection and “Find the Best CAS” work, when each is used, and how to configure your Client Access Servers for different scenarios.
Read more at source: http://msexchangeteam.com/archive/2007/09/04/446918.aspx