C# winform 邮件发送 这样的异常

C# winform 邮件发送 这样的错误

C# code

using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;using System.Net;using System.Net.Mail;namespace MyEmail{    public partial class EmailMain : Form    {        public EmailMain()        {            InitializeComponent();        }        /// <summary>        /// 全局变量 SmtpClient        /// </summary>        private SmtpClient sc = new SmtpClient();        string smtp = "";        private void EmailMain_Load(object sender, EventArgs e)        {        }        private void button3_Click(object sender, EventArgs e)        {            try            {                MailMessage Email = new MailMessage();                Email.To.Clear();                Email.From = new MailAddress(textBox1.Text.Trim() + smtp.Substring(4));                Email.To.Add(textBox3.Text.Trim());                Email.Subject = textBox4.Text;                Email.SubjectEncoding = Encoding.UTF8;                Email.Body = textBox6.Text;                Email.BodyEncoding = Encoding.UTF8;                Email.IsBodyHtml = true;                sc.Send(Email);//在这个地方出的错误。                MessageBox.Show("邮件已发送！","发送提示");            }            catch (Exception ex)            {                ex.Message.ToString();                MessageBox.Show("网络状况不好，发送失败！","发送提示");            }        }        #region --邮件发送另一方法--        /*        EmailParms mailstr = new EmailParms();        mailstr.EmailBody = "aaaa";        mailstr.EmailHostName = "smtp.XXXX.com";        mailstr.EmailPersonName = "admin";        mailstr.EmailPort = 25;        mailstr.EmailPriority = "high";        mailstr.EmailSubject = "主题";        mailstr.EncodingType = "UTF8";        mailstr.FromEmailAddress = "a@a.com";        mailstr.FromEmailPassword = "1111111";        mailstr.isBodyHtml = false;        mailstr.isEnableSsl = false;        mailstr.ToEmailAddress = "b@b.com";        sendmail.SendingEmail(mailstr);         */        #endregion        #region -----登录验证-----        /// <summary>        /// 登录验证        /// </summary>        /// <param name="UserName">用户名</param>        /// <param name="Pwd">密码</param>        /// <returns>验证结果</returns>        private  bool SetUserInfo(string UserName, string Pwd, string host, int port)        {            try            {                NetworkCredential NC = new NetworkCredential(UserName, Pwd);                sc.Host = host;                sc.Port = port;                sc.Timeout = 1000000;                sc.Credentials = new System.Net.NetworkCredential(UserName,Pwd);                return true;            }            catch(Exception e)            {                e.Message.ToString();                return false;            }        }        #endregion        private void button1_Click(object sender, EventArgs e)        {            if (textBox1.Text.Trim().Equals(""))            {                MessageBox.Show("请输入用户名！","验证提示");            }            if (textBox2.Text.Trim().Equals(""))            {                MessageBox.Show("请输入密码！", "验证提示");            }            if (textBox2.Text.Trim().Length < 6)            {                MessageBox.Show("密码输入不正确！", "验证提示");            }            if (!comboBox1.SelectedItem.ToString().Equals(""))            {                switch (comboBox1.SelectedIndex)                {                    case 0:                        smtp = "smtp@qq.com";                        break;                    case 1:                        smtp = "smtp@163.com";                        break;                    case 2:                        smtp = "smtp@126.com";                        break;                    default:                        break;                }            }            else            {                MessageBox.Show("请选择邮件服务器！", "验证提示");            }            bool isTrue = SetUserInfo(textBox1.Text.Trim(), textBox2.Text.Trim(), smtp, 25);            if (isTrue)            {                MessageBox.Show("登录成功！", "登录提示");            }            else            {                MessageBox.Show("登录失败,用户名或密码错误！", "登录提示");            }        }    }}