If a users mailbox is located on Exchange 2003 the Exchange 2010 CAS can silently redirect the user’s browser session to https://legacy.contoso.com/exchange using a hidden FBA form with the fields populated. OWA will return a small web page containing a hidden form with the same information as what the user had originally submitted to CAS2010 FBA page (username, password, public/private selector, URL to redirect to after logon) and a submit URL synthesized from The Exchange2003URL property defined on the CAS2010 OWA virtual directory, and target Exchange -specific path and query string. The web page will also contain script to automatically submit the form as soon as it is loaded. This is the last part of the logon process that CAS2010 will have a role in. Afterwards, no remnant of the user session should stick around.
FE2003 will consume that hidden form’s data, authenticate the user and proxy the request to the Exchange 2003 mailbox server and provide the data view back to the user. The response will contain an FBA cookie for the legacy namespace, and from that point on all user activity within the session will go to the legacy FE only.
To set the the exchange2003URL property on the exchange 2010 OWA virtual directory run the following in EMS:
Set-OwaVirtualDirectory “owa (Default Web Site)” -Exchange2003Url “https://exch2003.mydomain.com/exchange”
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