Outlook Web Access and Exchange 2007, Exchange 2003, and Exchange 2000 Coexistence
Please find this article which explains how Microsoft Outlook Web Access works in Exchange organizations that include computers that are running one or more versions of Microsoft Exchange that were released earlier than Microsoft Exchange Server 2007. These earlier versions can include Microsoft Exchange Server 2003 and Microsoft Exchange 2000 Server. You can use this information to plan a successful coexistence strategy. You can also use it to complete a successful migration to Exchange 2007.