how to send email after successfully registered in asp.net

try this one..

protected void btnSend_Click(object sender, EventArgs e)
{
try
{
System.Net.Mail.MailMessage mailMessage = new System.Net.Mail.MailMessage(txtMailFrom.Text, txtMailTo.Text, txtSubject.Text, txtBody.Text);
/* In case of adding CC and BCC
MailAddress bcc = new MailAddress(txtBcc.Text);
MailAddress cc = new MailAddress(txtCC.Text);
mailMessage.Bcc.Add(bcc);
mailMessage.CC.Add(cc);
*/
SmtpClient client = new SmtpClient();
client.EnableSsl = true;
client.Send(mailMessage);
Response.Write("<script>alert('Mail Sent')</script>");
}
catch (Exception ex)
{
throw ex;
}
}

And in the Configuration file add this

<system.net>

        <mailSettings>
            <smtp>
                <network host="" port="" userName="" password=""/>
            </smtp>
        </mailSettings>
    </system.net>

Use  ‘using System.Net.Mail;’

      /// <summary>
      /// Method to send mail.
     /// </summary>
      /// <param name="mailTo">Recipient's Email-Id</param>
      /// <param name="User">Recipient's User Name </param>
      /// <returns>true if mail sent; otherwise, false.</returns>
      private Boolean SendMail(String mailTo, String User)
      {
          Boolean sendMailResult;
          try
          {
              SmtpClient smtpServer = new SmtpClient();
              smtpServer.Credentials = new System.Net.NetworkCredential("", ""); //Add sender’s Email-Id and Password here.
              smtpServer.Port = 25;
              smtpServer.Host = "smtp.gmail.com";
              MailMessage alertMail = new MailMessage();
              alertMail.From = new MailAddress(""); //Add Sender’s Email-Id.
              alertMail.Subject = "Alert Mail";
              alertMail.To.Add(mailTo);
              alertMail.Body = "Dear " + User + "," + Environment.NewLine + " Please Check";
              smtpServer.Send(alertMail);
              sendMailResult = true;
              return sendMailResult;

          }
          catch (Exception ex)
          {
              string exceptionMessage=ex.Message;
              sendMailResult = false;
              return sendMailResult;
          }
          finally
          {

          }

      }