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.

  • For Outlook Web Access, Exchange 2010 CAS does not support rendering mailbox data from legacy versions of Exchange.  Exchange 2010 CAS does one of four scenarios depending on the target mailbox's version and/or location:
    • If the Exchange 2007 mailbox is in the same AD Site as CAS2010, CAS2010 will silently redirect the session to the Exchange 2007 CAS.
    • If the Exchange 2007 mailbox is in another Internet facing AD Site, CAS2010 will manually redirect the user to the Exchange 2007 CAS.
    • If the Exchange 2007 mailbox is in a non-Internet facing AD site, CAS2010 will proxy the connection to the Exchange 2007 CAS.
    • If the mailbox is Exchange 2003, CAS2010 will silently redirect the session to a pre-defined URL.
  • For Exchange ActiveSync, Exchange 2010 CAS does not support rendering mailbox data from legacy versions of Exchange.  Exchange 2010 CAS does one of four scenarios depending on the target mailbox's version and/or location, and device capabilities:
    • If the Exchange 2007 mailbox is in the same AD Site as CAS2010 and the device supports Autodiscover, CAS2010 will notify the device to synchronize with CAS2007.
    • If the Exchange 2007 mailbox is in the same AD Site as CAS2010 and the device does not support Autodiscover, CAS2010 will proxy the connection to CAS2007.
    • If the Exchange 2007 mailbox is in a non-Internet facing AD site, CAS2010 will proxy the connection to the Exchange 2007 CAS.
    • If the mailbox is Exchange 2003, CAS2010 will proxy the connection to the Exchange 2003 mailbox server.
  • For Outlook Anywhere, you are going to move the Outlook Anywhere endpoint from the Exchange 2003 Front-End or Exchange 2007 CAS to the Exchange 2010 CAS.  Exchange 2010 CAS will always proxy the Outlook MAPI RPC data that is embedded in the RPC-HTTPS packet to the target legacy mailbox server (regardless of AD site or version) or to the appropriate Exchange 2010 CAS