If you work with Outlook Web Access (OWA) along with Exchange Server 2000 or/and Exchange Server 2003 or/and Exchange Server 2007, then please find the wonderful technet article, which i read frequently and distribute it with my customers.
This article 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.