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Please find the articles how you can configure EWS impersonation for the Exchange 2010:
Hope this helps!!!
When EWS Impersonation is used the X-AnchorMailbox always should be correctly set to the mailbox being accessed. Without doing so you may get 500 or 503 errors at times. It is critical for performance and also for notifications. Not setting it can double or more the time it takes to complete the call. In some cases you can also get timeouts.
Example: service.HttpHeaders.Add("X-AnchorMailbox", targetSmtp);