Update 04/23/12: we have updated this blog post to indicate that fixes for this issue have been released. Please see below table or our announcements for release of Exchange 2010 SP2 RU2 and Exchan...
Updated Jul 01, 2019
Version 2.0
Blog Post
@ Greg Taylor [msft]: Regarding "CAS is normally able to proxy to older versioned CAS across AD sites. So SP1 can proxy to RTM for example". For example, just tried to proxy from SP2 (w/o RU1) to SP1 (no RUs). The result in OWA: "Outlook Web App isn't available. If the problem continues, please contact your helpdesk."
And the event is in the App log. So what should I test next to prove either your or my opinion? :)
Log Name: Application
Source: MSExchange OWA
Date: 2/20/2012 4:46:19 PM
Event ID: 136
Task Category: Proxy
Level: Error
Keywords: Classic
User: N/A
Computer: EXCAS1.qwerty. qwerty
Description:
The sign-in to Outlook Web App failed. User /o= qwerty /ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user4 has a mailbox on 14.1.218.0 server version. However, no Client Access server or front-end server with a matching version was found to handle the request.
For users on Exchange 2007 and above, you can configure the Outlook Web Access URL for redirection using the externalURL parameter on the Exchange /owa virtual directory.