MailMessage不工作，甚至与system.net.mail引用

MailMessage不工作，我不知道我错过了什么。请纠正我的程序，如果有缺失的代码或我使用错误的命名空间?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.Net.Mail;
using System.Web.Mail;
using System.Data.SqlClient;
using System.Data;
protected void btnsend_Click(object sender, EventArgs e)
{
   try
   {
      SmtpClient smtp=new SmtpClient("smtp.gmail.com",587);
      smtp.EnableSsl = true;
      smtp.UseDefaultCredentials = false;
      smtp.Credentials=new NetworkCredential(txtemail.Text,txtpassword.Text);
      MailMessage mail = new MailMessage(txtemail.Text, ddlemail.Text, txtsubject.Text, txtbody.Text);
      smtp.Send(mail);
   }
   catch (Exception ex)
   {
      Console.WriteLine("{0} Exception caught.", ex);
   }
}

using System.Net.Mail;
using System.Web.Mail;