Hi, the problem was that OWA was not redirecting https://ExCASServerName/Exchange to /OWA, which is the expected behaviour, and important because there are EX03 mailbox servers in the org.The response would be "The user name or password that you entered is not valid" or "...Access is denied due to invalid credentials"Tried troubleshooting with diagnostic logging, removing and recreating the OWA Virtual Directories and finally reinstalling the CAS role but nothing worked.Read through "Overview of Exchange Server 2007 CAS Proxying and Redirection" http://msexchangeteam.com/archive/2007/09/04/446918.aspx and noticed "Proxying and Redirection both do not support virtual directories that use Basic authentication"I checked the legacy virtual directory settings on the EX07 mailbox server and found that Basic Authentication and Windows Authentication weren't even installed as role services on the mailbox server's IIS. Installed and enabled on the OWA virtual directories on the mailbox server and the /Exchange to /OWA redirection started working.Hope this helps someone.regards,soon