An ActiveSync user cannot access a mailbox in an Exchange Server 2010 forest

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Symptoms

Consider the following scenario:

You have two Microsoft Exchange Server 2010 forests that have a trust relationship.
You use the Exchange Management Console or the Exchange Management Shell to configure an address in the ExternalURL property in the Microsoft Server ActiveSync virtual directory. This property enables redirection between the two forests.
An ActiveSync user uses a device to access a mailbox that is located in the remote forest. The device uses a version of Exchange ActiveSync (EAS) protocol that is earlier than EAS 12.1.
The Exchange Server 2010 Client Access Server (CAS) server returns an HTTP 451 redirect response. The response contains an address that redirects the user to the CAS that is located in the same forest as the mailbox.

In this scenario, the device does not recognize the HTTP 451 redirect response, and the redirection is not successful.

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Cause

This issue occurs because the protocol version of the request is logged incorrectly on the CAS server.

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Resolution

To resolve this issue, install the following cumulative update:

2661854 Description of Update Rollup 2 for Exchange Server 2010 Service Pack 2

After the update is installed, the CAS server acts as a proxy for requests from versions of EAS protocol that are earlier than EAS 12.1. Additionally, the CAS server redirects requests from versions of EAS that are later than EAS 14.0.

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