If you are planning on co-existing Exchange Server 2003 backend servers alongside Exchange Server 2007 mailbox servers, you will have the following question lingering in your mind:
But what about Outlook Web Access?
If some of your users have mailboxes on Exchange Server 2003 while others have mailboxes on Exchange Server 2007, and you publish your OWA URL as http://webmail.mycompany/owa, you will notice that Exchange 2003 users get the following error:
Outlook Web Access could not find a mailbox for DOMAINUSER. If the problem continues, contact technical support for your organization and tell them the following: The mailbox may be stored on a Microsoft Exchange 2000 or Microsoft Exchange 2003 server, or the Active Directory user account was created recently and has not yet replicated to the Active Directory site where this Client Access server is hosted.
The solution is to use the URL http://webmail.mycompany/exchange on the Client Access server. The ‘/exchange’ virtual directory on the Client Access server is able to proxy OWA requests for Exchange 2003 mailboxes to the appropriate Exchange 2003 back end server and the user sees the Exchange 2003 OWA experience. The ‘/exchange’ virtual directory will also automatically redirect OWA requests for Exchange 2007 mailboxes to the ‘/owa’ virtual directory.
For more information, see this post on the Exchange Team blog.