In Exchange 2007 we introduced the concept of proxying and redirection to manage the multi sites deployments (Internet facing & Non-Internet facing). In transitioning to Exchange 2010 CAS it’s a must now to use a new legacy Exchange environment with a new URL for OWA & EAS “mostly we use legacy.contoso.com” and which Exchange 2010 will use to serve the non-migrated mailboxes OWA/EAS requests through either redirection or proxying, so the concept of proxying and redirection is still there however they play a much more important role in a coexistence phase with Exchange 2003 or Exchange 2007. Ross posted a great article for transitioning CAS to Exchange 2010 at http://msexchangeteam.com/archive/2009/11/20/453272.aspx and which explain in depth the role of both in the transitioning process. To make it simpler there is a summary at the end of the article which I’m copying below and which is listing when we do proxying and when we do redirect.
To understand why we are introducing a new namespace for the legacy Exchange environment, it is important to understand what the Internet client behavior will be by introducing Exchange 2010.