Why do I sendMail fail in Win2008 R2,but success in My xp and win7 System with the same project?

   textBoxSmtpServer.Text = "smtp.gmail.com";
         textBoxSend.Text = "lisiname@gmail.com";
         textBoxDisplayName.Text = "李斯";
         textBoxPassword.Text = "lisiname***";
         textBoxReceive.Text = "mytestname@126.com";
         textBoxSubject.Text = "测试mytest";
         textBoxBody.Text = "This is a test（测试）";
         radioButtonSsl.Checked = true;

MailMessage mailMessage = new MailMessage();
         mailMessage.From = new MailAddress(textBoxSend.Text, textBoxDisplayName.Text, System.Text.Encoding.UTF8);
         mailMessage.To.Add(textBoxReceive.Text);
         mailMessage.Subject = textBoxSubject.Text;
         mailMessage.SubjectEncoding = System.Text.Encoding.Default;
         mailMessage.Body = textBoxBody.Text;
         mailMessage.BodyEncoding = System.Text.Encoding.Default;
         mailMessage.IsBodyHtml = false;
         mailMessage.Priority = MailPriority.Normal;
 SmtpClient smtpClient = new SmtpClient();
         smtpClient.Host = textBoxSmtpServer.Text;
         smtpClient.Port = 25;
                 smtpClient.EnableSsl = radioButtonSsl.Checked;
                   smtpClient.UseDefaultCredentials = false;
         smtpClient.Credentials = new NetworkCredential(textBoxSend.Text, textBoxPassword.Text);
                   smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
         try
         {
             smtpClient.Send(mailMessage);
                       }
         catch (SmtpException smtpError)
         {
             MessageBox.Show("发送失败:" + smtpError.StatusCode
                 + "\n\n" + smtpError.Message
                 + "\n\n" + smtpError.StackTrace);
         }
         finally
         {
             mailMessage.Dispose();
             smtpClient = null;
             this.Cursor = Cursors.Default;
         }