SMTP 服务器要求安全连接或客户端未经过身份验证

SMTP 服务器要求安全连接或客户端未通过身份验证。
protected void Cmd_Reg_Click(object sender, EventArgs e)
  {
  Random random = new Random();
  string checkCode = "";
  for (int i = 0; i <= 4; i++)
  {
  int num = random.Next(1, 10);
  checkCode += num.ToString();
  }
  string validataCode = FormsAuthentication.HashPasswordForStoringInConfigFile(checkCode, "md5");
  SqlConnection conn = new SqlConnection(str);
  conn.Open();
  string sql = "insert into Email(UserId,UserPas,mail,AddDate,ValidataCode,UserState) values(@UserId,@UserPas,@mail,@AddDate,@ValidataCode,@UserState)";
  SqlCommand cmd = new SqlCommand(sql,conn);
  SqlParameter[] param = { 
  new SqlParameter("@UserId",Txt_Name.Text),
  new SqlParameter("@UserPas",Txt_Pwd.Text),
  new SqlParameter("@mail",Txt_Email.Text),
  new SqlParameter("@AddDate",DateTime.Now),
  new SqlParameter("@ValidataCode", validataCode),
  new SqlParameter("@UserState",false),
  };
  cmd.Parameters.AddRange(param);
  int result = cmd.ExecuteNonQuery();
  conn.Close();
  if (result > 0)
  {
  ClientScript.RegisterStartupScript(this.Page.GetType(), "alert", "<script>alert('ok')</script>");
  string strSmtpServer = "smtp.gmail.com";
  string strFrom ="358730096@qq.com";
  string strFromPass = "123456";
  string strto = Txt_Email.Text;
  string strSubject = "账号激活";
  StringBuilder strBody = new StringBuilder();
  strBody.Append("点击下面链接激活账号，48小时生效，否则就要重新注册账号，链接只能使用一次，请尽快激活！</br>");
  strBody.Append("<a href='http://localhost:2493/web/Operate.aspx?userName=" + Txt_Name.Text + "&validateCode=" + validataCode + "'>点击这里</a></br>");
  strBody.Append("如未能激活请点击下面链接：<a href='http://localhost:2493/web/Operate.aspx?userName=" + Txt_Name.Text + "&validateCode=" + validataCode + "'>http://localhost:2493/web/Operate.aspx?userName=" + Txt_Name.Text + "&validateCode=" + validataCode + "</a></br>");
  SendSMTPEMail(strSmtpServer, strFrom, strFromPass, strto, strSubject, strBody.ToString());
  }
  else
  {
  ClientScript.RegisterStartupScript(this.Page.GetType(), "alert", "<script>alert('Nok')</script>");
  }  
  }
  //发送邮件
  public void SendSMTPEMail(string strSmtpServer, string strFrom, string strFromPass, string strto, string strSubject, string strBody)
  {  
  MailMessage message = new MailMessage(strFrom, strto, strSubject, strBody);
  message.BodyEncoding = System.Text.Encoding.UTF8;
  message.IsBodyHtml = true;
  message.Priority = MailPriority.High;
  SmtpClient client = new SmtpClient(strSmtpServer);