We are running in coexistence with Exchange 2010 and 2003. I have set up the legacy namespace for OWA access and initially we were getting an "unspecified error" message when attempting to connect to OWA 2003 through our CAS array. I found a TID that contained the fix, enabling forms based authentication on the 2003 OWA Exchange Virtual Server. After enabling FBA we could connect to both 2010 and 2003 accounts through the CAS array using a web browser. We also verified that our Exchange 2010 users with Android, iPhone and tablet devices could use their mail client with the external URL that we use for OWA set as their mail server (mail.xxxx.com). However we found that our 2003 mailbox users got a pop-up window asking for their password, entering their network password did not work. We tried recreating an iPhone user's mail profile and after entering her credentials she got a message that the server was unable to validate authentication.
I'm wondering if this is because we enabled FBA on the 2003 OWA server but I'm not sure how to resolve this. We need our 2003 users to be able to access OWA through a web browser and still be able to use their mobile device mail clients.