EWS(Exchange Web服务)获取未经授权的错误

我在尝试发送或阅读电子邮件时遇到未经授权的(401)错误

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);
            service.TraceEnabled = true;
            service.TraceFlags = TraceFlags.All;

            service.Credentials = new WebCredentials("XXXX@YYY.COM", "XXXXXXXX");
            service.AutodiscoverUrl("XXXXXX@YYYYY.com", RedirectionUrlValidationCallback);

 EmailMessage email = new EmailMessage(service);
            email.ToRecipients.Add("abc@yahoo.com");
            email.Subject = "Testing from C#";
            email.Body = new MessageBody("This is the first email I've sent by using the EWS Managed API");
            email.Send();