Outlook Anywhere clients cannot connect to a CAS server after you apply Exchange Server 2010 SP2 RU3

Consider the following scenario:

• A Microsoft Exchange Server 2010 organization has two sites.
• Outlook Anywhere is enabled on the Client Access (CAS) servers in Site A.
• The CAS servers in Site A are exposed to the Internet.
• Outlook Anywhere is enabled on the CAS servers in Site B.
• The CAS servers in Site B are not exposed to the Internet.
• You install Microsoft Exchange Server 2010 Service Pack 2 (SP2) update Rollup 3 (RU3).

In this scenario, Outlook Anywhere clients cannot connect to their mailboxes. An error message that resembles the following is logged in the RPC Client Access log:

This issue occurs because of a change in Exchange Server 2010 SP2 RU3. After you install Exchange Server 2010 SP2 RU3, RPC clients are redirected to the CAS server in the same site as the client mailbox.

Note CAS servers in other sites will not accept an RPC connection request if there is an endpoint that is Outlook Anywhere enabled in the same site as the mailbox. Additionally, the client profile is updated.

To resolve this issue, use one of the following methods, as appropriate for your situation.

Method 1: Disable Outlook Anywhere on CAS servers that are not exposed to the Internet

Note We recommend that you use this method to resolve the issue that is described in the "Symptoms" section.

To resolve this issue, disable Outlook Anywhere on CAS servers that are not exposed to the Internet. After you disable Outlook Anywhere on CAS servers that are not exposed to the Internet, clients can connect to a CAS server in a different site.

Method 2: Disable preferred site enforcement

To resolve this issue, disable preferred site enforcement. To do this, create the following DWORD registry value:

Key:
Value Name: EnablePreferredSiteEnforcement
Data Type: REG_DWORD
Value: 0

Note After you set the EnablePreferredSiteEnforcement value to 0, a CAS server will accept connection requests from any site and not try to redirect the request to a different server.

In earlier versions of Exchange Server 2010, client connection requests were not redirected to other CAS servers. Therefore, the endpoints were not used in a site that was not exposed to the Internet. The CAS server to which the client connected would connect directly to the mailbox server. Additionally, the user profiles were not updated. Because the user profiles were not updated, availability issues might occur if a mailbox was moved to a different site.

After you install Exchange Server 2010 SP2 RU3, Exchange Server tries to redirect connection requests and update the user profile intelligently. This can result in better performance and increased availability.

For more information about cross-site availability issue in earlier versions of Exchange Server 2010, visit the following TechNet website: