Exchange 2010 Client Access behavior in coexistence with Exchange 2003 and 2007

I answered a post on Technet regarding this so I thought I would post it here as well.

There is no possibility to replace a Exchange 2007 CAS server with a Exchange 2010 CAS server. Exchange 2010 CAS does not support rendering mailboxes from legacy versions of Exchange. In the scenarios below I have included both Exchange 2003 and 2007.

For OWA:

  • When the Exchange 2007 mailbox is in the same AD Site as Exchange 2010 CAS, Exchange 2010 CAS will silently redirect the session to the Exchange 2007 CAS.
  • When the Exchange 2007 mailbox is in another Internet facing AD Site, Exchange 2010 CAS will manually redirect the user to the Exchange 2007 CAS.
  • When the Exchange 2007 mailbox is in a non-Internet facing AD site, Exchange 2010 CAS will proxy the connection to the Exchange 2007 CAS.
  • When the server is running Exchange 2003, Exchange 2010 CAS will silently redirect the session to a pre-defined URL.

For ActiveSync:

  • When the Exchange 2007 mailbox is in the same AD Site as Exchange 2010 CAS and the device supports Autodiscover, Exchange 2010 CAS will notify the device to synchronize with Exchange 2007 CAS.
  • When the Exchange 2007 mailbox is in the same AD Site as Exchange 2010 CAS and the device does not support Autodiscover, Exchange 2010 CAS will proxy the connection to Exchange 2007 CAS.
  • When the Exchange 2007 mailbox is in a non-Internet facing AD site, Exchange 2010 CAS will proxy the connection to the Exchange 2007 CAS.
  • When the server is running Exchange 2003, Exchange 2010 CAS will proxy the connection.

For Outlook Anywhere:

When migrating Outlook Anywhere you move the Outlook Anywhere endpoint from the Exchange Exchange 2007 CAS to the Exchange 2010 CAS. Exchange 2010 CAS will then proxy the Outlook MAPI RPC to either the Exchange 2007 Mailbox server or the Exchange 2010 Mailbox server depending on the mailbox home server.

So in all scenarios for OWA and Activesync you would still need the Exchange 2007 CAS to handle requests for Exchange 2007 mailboxes. Thanks for reading and as usual, please let me know if you have any further questions!

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