We have one Exchange 2003 server and will be bringing a single Exchange 2007 server into the Org. We will migrate mailboxes slowly. During the migration is it possible for a user with a mailbox on the 2003 box to access OWA on the 2007 server or does the mailbox absolutely have to reside on the 2007 server. In my lab, I get an error when the 2003 user tries to connect to the 2007 OWA stating the mailbox does not reside on this server. Thanks

Ordinarily your Exchange 2003 users would type https://e2k7-cas/exchange, which would re-direct the OWA session to the back-end Exchange 2003 server. However, if you have the CAS role coexisting with the Mailbox role on your Exchange 2007 server, this redirection won't work. Therefore, my advice to you is (if OWA coexistence is important) to bring up a temporary Exchange 2007 server just running the CAS role and point all users to that.

If I add a stand alone CAS do I need to remove that role from my mailbox server? Is it a difficult task? Thanks

Hey, Please refer this article this might be helpful. http://msexchangeteam.com/archive/2007/02/07/434523.aspx

I'd probably leave the CAS role on the mailbox server and point the users to the new CAS. Once you've migrated, the redirection problem will go away and therefore you can remove the temporary CAS server (hence why I'd leave the CAS role on the original server).

Yes, in co-existence environment, we'd better to install CAS role and Mailbox on separate machine. Detail information you can refer to the article bleow: Outlook Web Access and Exchange 2007, 2003 and 2000 coexistence http://msexchangeteam.com/archive/2007/02/07/434523.aspx Regards, Xiu