Consider the following scenario:

  • Users connect to their mailbox using an EWS client.

  • The user's mailbox is located in Site B.

  • The client connection is made to an Exchange server in Site A.

  • The Exchange server in Site A proxies the request to an Exchange server in Site B.

In this scenario, the user is unable to send an email with large attachments. When using the Outlook for Mac client, it moves the message into the drafts folder and returns the following error:

An unknown error has occurred. Connection to the server failed or was dropped. Outlook cannot send the message, 
because the message size exceed the maximum limit set on the server

The EWS logs in Site A show an HTTP 500 status with a generic error similar to the following:

ReportException_Message=Exchange Web Services are not currently available for this request because none of 
the Client Access Servers in the destination site could process the request.;ReportException_StackTrace= at
Microsoft.Exchange.Services.Wcf.EWSService.GetServiceAsyncResult[TSoapResponseBody](IAsyncResult result) at
result; Func`2 createSoapResponseCallback) at AsyncInvokeEndEndCreateItem(Object ; Object[] ; IAsyncResult )
at System.ServiceModel.Dispatcher.AsyncMethodInvoker.InvokeEnd(Object instance; Object[]& outputs;
IAsyncResult result) at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeEnd(MessageRpc& rpc)
at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage7(MessageRpc& rpc) at
System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet);

The EWS logs in Site B show an HTTP 500 status with a generic error similar to the following:

ReportException_Message=The remote host closed the connection. The error code is 0x80070057.;
ReportException_StackTrace= at System.ServiceModel.Channels.HttpOutput.HostedRequestHttpOutput.HostedResponseOutputStream.CheckWrapThrow(Exception e) at
System.ServiceModel.Channels.HttpOutput.HostedRequestHttpOutput.HostedResponseOutputStream.Write(Byte[] buffer; Int32 offset; Int32 count) at
System.IO.BufferedStream.FlushWrite() at System.IO.BufferedStream.Flush() at System.Xml.XmlStreamNodeWriter.Flush() at
Microsoft.Exchange.Services.Wcf.SoapWcfResponseRenderer.Render(Message message; Stream stream) at
Microsoft.Exchange.Services.Wcf.MessageEncoderWithXmlDeclaration.WriteMessage(Message message; Stream stream) at
System.ServiceModel.Channels.HttpOutput.WriteStreamedMessage(TimeSpan timeout) at System.ServiceModel.Channels.HttpOutput.Send(TimeSpan timeout) at
System.ServiceModel.Channels.HttpRequestContext.OnReply(Message message; TimeSpan timeout) at
System.ServiceModel.Channels.RequestContextBase.Reply(Message message; TimeSpan timeout) at
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.Reply(MessageRpc& rpc);


The connection between the Exchange servers in the two sites for this request is dropped based on a ProxyTimeout setting.


To resolve this issue, increase the ProxyTimeout value on the CAS server by editing the web.config file for EWS. To do this, follow these steps:

  1. Locate the web.config file and open the file by using Notepad or another text editing program. The default location is %ExchangeInstallPath%\ClientAccess\exchweb\ews.

  2. In the file, scroll down the text to find the following statement: <AppSettings>

  3. Add the following line within the <AppSettings> section to increase the timeout:
     <add key="ProxyTimeout" value="120000" />

  4. Restart the MSExchangeServicesAppPool application pool.

Note: The default value for the ProxyTimeout is 59000. This value should be increased gradually until the issue is resolved. For example, start by increasing the timeout to 120000 and if the issue persists, increase the value to 210000.

More Information

Start or Stop an Application Pool

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