Earlier I created a delegate in a Exchange Server 2007 environment. I tried creating, sending and then updating meeting requests for the organizer by using Exchange Web Service (EWS). However, the operation does not work if the delegate tries to cancel a meeting, and then tries to send the cancellation update request for the organizer by using EWS. In this scenario, when the delegate tries to cancel a meeting for the organizer, an "ErrorItemNotFound" error is returned.
To resolve this problem, install the Update Rollup 1 for Exchange Server 2007 Service Pack 2. You can download it from the following: Exchange2007-KB971534-EN package