In 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.
Understanding CAS Proxying: In Exchange 2003, the front-end server communicates with the back-end server over HTTP. In Exchange 2007, the Client Access Server communicates with the mailbox server over RPC.
It is a requirement to have a Client Access Server in each site where there is an Exchange 2007 Mailbox Server. The recommendation is to have the Client Access Server as the first Exchange 2007 Server role installed in each Active Directory site. If you were to just have a Mailbox Server role in any given site without a Client Access Server no users would be able to connect to their mailboxes via Outlook Web Access, ActiveSync, Exchange Web Services, POP3 and IMAP4.
The Client Access Server can be configured for internal access or can be Internet-facing named "First CAS". If there is no Internet-facing Client Access Server in the same site as the mailbox, then the request will be proxied from the Internet-facing Client Access Server to the internal Client Access Server named "Second CAS". All the traffic between First CAS and Second CAS is over http(s).