电子邮件确认Asp.net错误

我正在做注册时的电子邮件确认。

面对这个错误信息:附加信息:SMTP服务器需要安全连接或客户端未经过身份验证。服务器响应为:5.5.1 Authentication Required。在

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Configuration;
using System.Data.SqlClient;
using System.Net.Mail;
using System.Net;
public partial class CS : System.Web.UI.Page
{
protected void RegisterUser(object sender, EventArgs e)
{
    int userId = 0;
    string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("Insert_User"))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Username", txtUsername.Text.Trim());
                cmd.Parameters.AddWithValue("@Password", txtPassword.Text.Trim());
                cmd.Parameters.AddWithValue("@Email", txtEmail.Text.Trim());
                cmd.Connection = con;
                con.Open();
                userId = Convert.ToInt32(cmd.ExecuteScalar());
                con.Close();
            }
        }
        string message = string.Empty;
        switch (userId)
        {
            case -1:
                message = "Username already exists.''nPlease choose a different username.";
                break;
            case -2:
                message = "Supplied email address has already been used.";
                break;
            default:
                message = "Registration successful. Activation email has been sent.";
                SendActivationEmail(userId);
                break;
        }
        ClientScript.RegisterStartupScript(GetType(), "alert", "alert('" + message + "');", true);
    }
}
private void SendActivationEmail(int userId)
{
    string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    string activationCode = Guid.NewGuid().ToString();
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("INSERT INTO UserActivation VALUES(@UserId, @ActivationCode)"))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.AddWithValue("@UserId", userId);
                cmd.Parameters.AddWithValue("@ActivationCode", activationCode);
                cmd.Connection = con;
                con.Open();
                cmd.ExecuteNonQuery();
                con.Close();
            }
        }
    }
    using (MailMessage mm = new MailMessage("sender@gmail.com", txtEmail.Text))
    {
        mm.Subject = "Account Activation";
        string body = "Hello " + txtUsername.Text.Trim() + ",";
        body += "<br /><br />Please click the following link to activate your account";
        body += "<br /><a href = '" + Request.Url.AbsoluteUri.Replace("CS.aspx", "CS_Activation.aspx?ActivationCode=" + activationCode) + "'>Click here to activate your account.</a>";
        body += "<br /><br />Thanks";
        mm.Body = body;
        mm.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        NetworkCredential NetworkCred = new NetworkCredential("sender@gmail.com", "<password>");
        smtp.UseDefaultCredentials = false;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
    }
}

}

//This is my web config file

<?xml version="1.0"?>
<!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  -->
<configuration>
  <connectionStrings>
    <add name="constr" connectionString="Data Source=.'SQL2005;Initial Catalog=LoginDB;User id = sa;password=pass@123" />
    <add name="ConnectionString" connectionString="Data Source=(LocalDB)'v11.0;AttachDbFilename=|DataDirectory|'LoginDB.mdf;Integrated Security=True;Connect Timeout=30"
      providerName="System.Data.SqlClient" />
  </connectionStrings>
  <system.web>
    <compilation debug="true" targetFramework="4.0"/>
  </system.web>
</configuration>

<system.net>
    <mailSettings>
      <smtp from="fromAddress@domain.com">
        <network host="smtpEmailServerAddress" port="25" enableSsl="true"
        userName="username" password="password"/>
      </smtp>
    </mailSettings>
</system.net>