为什么在 System.Net.Mail 不能发送 esmtp 的邮件？

        private void button1_Click(object sender, EventArgs e)
        {
            NetworkCredential myNetworkCredential = new NetworkCredential("user", "pass");

            SmtpClient mail = new SmtpClient("host");

            mail.UseDefaultCredentials = false;
            mail.Credentials = myNetworkCredential;
            

            try
            {
                mail.Send("frommail", "tomail", "just a test", "test message");
                MessageBox.Show("发送成功");
            }
            catch (SmtpException ex)
            {
                SmtpStatusCode code = ex.StatusCode;
                MessageBox.Show(ex.Message);
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                
            }
        }

发送失败，郁闷~~~~