Hi, We have two AD sites. Site A is contoso.int and site B is us.contoso.int. Site A has a internet facing Exchange 2010 CAS called CAS01 published as mail.contoso.com. Recently I implemented a new Exchange 2010 server in site B: us.contoso.int with CAS configured as non-internet facing. From my understanding, CAS at site A should automatically proxy OWA traffic for users in Site B Mailbox server. However, this is not working. After put in us\user credential in mail.contoso.com/owa, I am getting error: "Outlook Web App isn't available. If the problem continues, please contact your helpdesk." I can confirm the OWA at site B us.contoso.int is working internally, if access directly by using the server name as https://uscas01.us.contoso.int/owa. I have since checked OWA-VirtualDirectory settings. Site A configured as: internalURL: https://CAS01.contoso.int/owa externalurl: https://mail.contoso.com/owa, externalAuthenticationMethods set to {Fba}. Site B configured as: internalURL: https://USCAS01.us.contoso.int/owa externalurl: null, externalAuthenticationMethods {Fba}.

Please make sure that in Site B, OWA is configured to use integrated windows authentication. Make sure that External URL under general tab is blank for Site B CAS please. Make sure that Site to site link is there & replication works fine, essentially site B shouldn’t be in island or so......Regards, Pushkal MishrA