A 404 error is generated in the Migration Agent for Exchange (MAgE) log similar to this:
2015-04-03 17:38:49.1655 PxFD4 TxE A108 C1 M982 Error Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed. The remote server returned an error: (404) Not Found. | at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse(IEwsHttpWebRequest request)
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest(IEwsHttpWebRequest& request)
at Microsoft.Exchange.WebServices.Data.SimpleServiceRequestBase.InternalExecute()
at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest`1.Execute()
at Microsoft.Exchange.WebServices.Data.ExchangeService.BindToFolder(FolderId folderId, PropertySet propertySet)
at Microsoft.Exchange.WebServices.Data.ExchangeService.BindToFolder[TFolder](FolderId folderId, PropertySet propertySet)