How Exchange Server 2007 CAS Proxying works for Outlook Web Access (OWA)
Now the subject is the CAS Proxying process with OWA.
Proxying for Outlook Web Access is used when there is only one Internet-facing CAS Server. In this scenario the will be proxied to the CAS server where his mailbox server is located. This requires more intensive resource on the CAS server than doing redirection. The main benefit to this is your users can have a single point of access from external and proxying is transparent to the end user.
CAS-CAS Proxying is quite similar with the process described in the future topic "How CAS proxying works for ActiveSync". There are a few differences regarding the errors and logs.
Read more at source: http://msexchangeteam.com/archive/2007/09/10/446957.aspx