private void btnSend_Click(object sender, EventArgs e)
        {
            IPAddress[] ipAddress = Dns.GetHostEntry(txtServerName.Text).AddressList;
            string ipadd = "";

            Ping ping= new Ping();
            PingReply pingReply =null;

            foreach (IPAddress ip in ipAddress)
            {
                pingReply = ping.Send(ip, Convert.ToInt32(txtPort.Text));
                if (pingReply.Status ==IPStatus.Success)
                {
                    ipadd = ip.ToString();
                    break;
                }
            }

           SmtpClient sc = new SmtpClient(ipadd.ToString(), Convert.ToInt32(txtPort.Text));
            sc.Credentials = new System.Net.NetworkCredential(txtUserName.Text, txtPassword.Text);

            //sc.Host = ipadd.ToString();
            //sc.Port = Convert.ToInt32(txtPort.Text);
            

            MailAddress fromMail = new MailAddress(txtFrom.Text, txtDisplay.Text);
            MailAddress toMail = new MailAddress(txtTo.Text);

            MailMessage ms = new MailMessage();
            ms.Body = txtContent.Text;
            ms.Subject = txtTitle.Text;
            ms.ReplyTo = fromMail;
            ms.From = fromMail;
            ms.Sender = fromMail;
            ms.To.Add(toMail);

            sc.SendCompleted += new SendCompletedEventHandler(SendCompleted);
            sc.Send(ms);//错误发生在这里  错误信息：SmtpException was unhandled            
        }
        private void SendCompleted(object sender, AsyncCompletedEventArgs e)
        {
            if (e.Cancelled)
            { MessageBox.Show("发送已取消！"); }
            if (e.Error != null)
            {

                MessageBox.Show(e.UserState.ToString() + "发送错误：" + e.Error.Message, "发送错误", MessageBoxButtons.OK, MessageBoxIcon.Error);

            }
            else
            {
                MessageBox.Show("邮件成功发出!", "恭喜!", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }


        }