Send Mail With Attachment
   protected void btnSubmit_Click(object  sender, EventArgs e)
  {
      try
     {
          MailMessage mail = new MailMessage();
          SmtpClient _smtpClient = new SmtpClient("smtpServer  IP or Name", PortNumber);
          mail.From = new MailAddress(txtFrom.Text.Trim());
          mail.To.Add("First Recipient Address");
          mail.To.Add("Second Recipient Address ");
          mail.Subject = txtSubject.Text.Trim();
          mail.Body = txtBody.Text.Trim();
          System.Net.Mail.Attachment attachment;
          attachment = new System.Net.Mail.Attachment("D://Test.txt");
          mail.Attachments.Add(attachment);
         _smtpClient.Credentials  = new System.Net.NetworkCredential("Email  ID", "password");
         _smtpClient.EnableSsl  = true;
        _smtpClient.Send(mail);
         ClientScript.RegisterStartupScript(typeof(Page), "Success",  "<script type=\"text/javascript\"> alert('Mail Sent Successfully  !!!');</script>");
     }
     catch  (Exception ex)
     {
            ClientScript.RegisterStartupScript(typeof(Page), "Exception",  "<script type=\"text/javascript\"> alert('Exception: " + ex.Message.Trim()  + "');</script>");
      }
  } 
 Send Mail Without Attachment
  SmtpClient _smtpClient  = new SmtpClient("smtpServer  IP or Name", PortNumber);
  _smtpClient.Credentials  = new System.Net.NetworkCredential("Email  ID", "password");
   string  from, to, sub, body;
  from =  txtFrom.Text.Trim();
  to =  txtTo.Text.Trim();
  sub =  txtSubject.Text.Trim();
  body =  txtBody.Text.Trim();
  _smtpClient.Send(from,  to, sub, body);