Outlook Web Access does not appear to be proxy friendly. It uses non-standard HTTP verbs. I observed SEARCH and SUBSCRIBE. SEARCH appears to be a old proposed extension to WebDAV. I can't find anything about SUBSCRIBE. Interestingly, the MS web sites don't seem to have any information about these verbs. OWA only appears to use these verbs when talking to IE -- when I tried FF, only standard HTTP verbs were used (and the application looks quite different).