Debug: Syncing custom EWS app to an Exchange Server 2010 mailbox receive ErrorServerBusy

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 error occurs when the server is busy. Exchange Web Services reports that the given mailbox cannot be accessed because of throttling limits imposed by Exchange. The BackOffMilliseconds value returned with ErrorServerBusy errors indicates to the client the amount of time it should wait until it should resubmit the request that caused the response that returned this error code.

In such scenario, consider the following:

- Make sure that your Exchange Server is fully patched – its mandatory. As i remember we fixed one at Exchange Server 2010 SP3 .
- Check your throttling limits
- Check your app code and make sure its optimized & wisely coded
- Refer EWS Throttling @ https://msdn.microsoft.com/en-us/library/office/jj945066(v=exchg.150).aspx 
- Refer https://support.microsoft.com/en-in/help/2800080/errorserverbusy-response-code-when-you-synchronize-an-ews-based-applic
- Check the values of Web.config “MbxServerEnableConcurrencyControl”, “``MdbFairUnhealthyLatencyThreshold”

Hope this helps.