We're experiencing a very frustrating issue with OWA access for users with mailboxes in Active Directory Sites that aren't in the CAS\Mailbox Exchange server (call it SiteA for convenience) that has the External URL parameter listed.
We are running Exchange 2010 SP1 at all sites and, after following the MS technet article - http://technet.microsoft.com/en-us/library/bb310763.aspx
, set up our servers so that our Server at SiteA that faces externally has the internal and external OWA URL listed and all the others at remote sites only have the internal URL listed, with External URL being left as blank. What should happen is that any user in any site navigating to https://mail.example.com/owa
should be proxied to their relevant mailbox server. However what happens is that users with mailboxes at SiteA work without an issue but users at any other site receive "Outlook Web App isn't available. If the problem continues, please contact your helpdesk".
Frustratingly, and quite bizarrely, Exchange Activesync works without any issues for users in all sites.
I get the feeling this is an issue related more towards IIS or Windows\Forms authentication on each of the target mailbox servers but I don't want to start guessing. Can anyone help shed any light on this issue?