c# EWS Delete Appointment不起作用

我想通过EWS删除Appointments，但它不起作用。我有以下代码:

private void deleteAppointment(object obj)
{
    ServicePointManager.ServerCertificateValidationCallback = CertificateValidationCallBack;
    ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2010_SP2);
    service.Credentials = new WebCredentials(CredentialCache.DefaultNetworkCredentials);
    service.AutodiscoverUrl("user@place.com", RedirectionUrlValidationCallback);
    ItemId ii = (ItemId)obj;
    Appointment a = Appointment.Bind(service, ii);
    try
    {
        a.Delete(DeleteMode.MoveToDeletedItems);
    }
    catch (ServiceResponseException ex)
    {
        MessageBox.Show(ex.Message);
    }
}

登录到机器(me)上的用户是来自user地址的日历的所有者。但是，如果我试图删除Appointment, Exception是"对象不能被删除"。ItemId是正确的。如果我使用user地址作为Credentials，它可以工作。