问请求失败，因为EWS无法就此请求与适当的CAS服务器联系

EN

EWS订阅正在丢失，intermittently.It抛出以下两个错误。

Microsoft.Exchange.WebServices.Data.ServiceResponseException:邮箱数据库暂时不可用。无法打开邮箱/o=ExchangeLabs/ou=管理组

Microsoft.Exchange.WebServices.Data.ServiceResponseException：错误2请求失败，因为无法就此请求与适当的服务器联系。( Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ProcessWebException(WebException webException) (应Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse(IEwsHttpWebRequest请求))(应Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest(IEwsHttpWebRequest&请求)( Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute() at Microsoft.Exchange.WebServices.Data.ExchangeService.SubscribeToStreamingNotifications(IEnumerable1 folderIds，EventType[] eventTypes)

我使用了Exchange 2013 exchange服务。

ExchangeService exchange = new ExchangeService(ExchangeVersion.Exchange2013);
                exchange.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, _primaryMailbox);
                exchange.HttpHeaders.Add("X-AnchorMailbox", _primaryMailbox);
                exchange.HttpHeaders.Add("X-PreferServerAffinity", "true");

下面是添加订阅的方法。

 ExchangeService exchange = group.ExchangeService;
                exchange.Credentials = new WebCredentials(BCCSettings.ImpersonatedUsername, BCCSettings.ImpersonatedPassword);
                exchange.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, mailbox);

                subscription = exchange.SubscribeToStreamingNotifications(PreparFolders().ToArray(), EventType.NewMail, EventType.Moved);

任何帮助都将不胜感激。