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Assume that you try to synchronize a Microsoft Exchange ActiveSync device with a Microsoft Exchange Server 2010 mailbox by using the Exchange ActiveSync service. However, if some external URLs are not resolved on the Exchange Server 2010 Client Access server role, you cannot synchronize the device. Additionally, the following exception occurs on the Exchange server that is running the Client Access server role:

404 03FDF57B Error Number MSExchangeSync Requests ProxyHandler.BackEndGetRequestStream caught an exception
--- Exception start ---
Exception type: System.Net.WebException
Exception message: The remote name could not be resolved: 'DomainName'
Exception level: 0
Exception stack trace: at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult)
at Microsoft.Exchange.AirSync.ProxyHandler.BackEndGetRequestStream(IAsyncResult asynchronousResult)
--- Exception end ---


This issue occurs because the Client Access server role cannot resolve the external URLs that are specified for the Exchange ActiveSync service. Therefore, the Autodiscover service cannot update the device profile that is stored in the mailbox on the Exchange Server 2010 servers.


To work around this issue, use one of the following methods.

Method 1

Make an internal Domain Name System (DNS) zone available to enable Exchange Server 2010 servers to resolve external URLs for the Exchange ActiveSync service and for Microsoft Outlook Web App.

Method 2

Create a forward lookup zone that has the same name as the external URL setting in the internal DNS configuration. Leave the host record blank, and enter the internal IP address of the Client Access server or of the Client Access server array in the IP address box.

Method 3

Create a host file that points to the external URL and to one of the following locations:

  • The internal IP address of the Client Access server

  • The internal IP address of the Client Access server array on the Client Access server


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

More Information

For more information about disjoint namespaces, visit the following Microsoft website:

General information about disjoint namespacesFor more information about how to create DNS records for the Autodiscover service, visit the following Microsoft website:

How to create DNS records for the Autodiscover service

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