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 Exchange 2007 SP3 RU7

We wanted to give you a heads up regarding a change in CAS to CAS proxy behavior between servers running Exchange 2010 SP2 RU1 and servers running older versions of Exchange.

The SP2 RU1 package introduced a change to the user context cookie which is used in CAS-to-CAS proxying. An unfortunate side-effect is a temporary incompatibility between SP2 RU1 servers and servers running earlier versions of Exchange. The change is such that earlier versions of Exchange do not understand the newer cookie used by the SP2 RU1 server. As a result, proxying from SP2 RU1 to an earlier version of Exchange will fail with the following error:

Invalid user context cookie found in proxy response

The server might show exceptions in the event log, such as the following:

Event ID: 4999
Log Name: Application
Source: MSExchange Common
Task Category: General
Level: Error
Description: Watson report about to be sent for process id: 744, with parameters: E12, c-RTL-AMD64, 14.02.0283.003, OWA, M.E.Clients.Owa, M.E.C.O.C.ProxyUtilities.UpdateProxyUserContextIdFromResponse, M.E.C.O.Core.OwaAsyncOperationException, 413, 14.02.0283.003.

Not all customers are affected by this. But since we received a few questions about this, we wanted to let you know about the change. Many Exchange customers do not use proxying between Exchange 2010 and Exchange 2007 but rather use redirection, which is not affected by the change. However, if you are using CAS-to-CAS proxying, where an Exchange 2010 SP2 RU1 Client Access server is proxying to an earlier version of Exchange 2010 or Exchange 2007 Client Access server, then you are affected by the change.

If you are affected after application of Exchange 2010 SP2 RU1, please see the below table for the solution to this problem.

The Exchange Team