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When you try to synchronize an Exchange Web Service (EWS)-based application to a mailbox in a Microsoft Exchange Server 2010 environment, you receive an ErrorServerBusy response code and an error message that resembles the following:

The server cannot service this request right now. Try again later.


This issue occurs because the EWS monitoring detects that the limit for the requests that are from a Client Access server to a Mailbox server was reached. Therefore, the Client Access server returns an ErrorServerBusy response code to all EWS requests to the Mailbox server.


To resolve this issue, install Exchange Server 2010 Service Pack 3 (SP3) on the Exchange Server 2010 servers. For more information about Exchange Server 2010 SP3, click the following article number to view the article in the Microsoft Knowledge Base:

2808208 Description of Exchange Server 2010 SP3


To work around this issue, make the following changes in the Web.config file on the Client Access server:

  1. Find the following code in the Web.config file:

    <add key="MbxServerEnableConcurrencyControl" value="true" />

    Change the MbxServerEnableConcurrencyControl value from true to false.

  2. Add the following line of code into the Web.config file:

    <add key="MdbFairUnhealthyLatencyThreshold" value="500" />

Note The Web.Config file is located in the following path:

Drive\Program Files\Microsoft\Exchange Server\V14\ClientAccess\exchweb\ews


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 the ErrorServerBusy response code, go to the following Microsoft website:

General information about response codes

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