SMTP Email Error: Unable to read data from the transport connection: net_io_connectionclosed

hi, can anyone help me, I can not send mail, this error means ?, Thank you for your help.

Code:

using (MailMessage mm = new MailMessage("soporte@linknred.com", correo1.Text.Trim()))
{
   mm.Subject = "Activacion de Cuenta";
   string body = "Hola " + correo1.Text.Trim() + ", " + "Bienvenido a LinknRed";
   body += "<br /><br />Por Favor haz Click en el Enlace Siguiente Para Activar Tu Cuenta";
   body += "<br /><a href = '" + Request.Url.AbsoluteUri.Replace("Clientes_Reg.aspx", "Clientes_Activacion.aspx?ActivationCode=" + activationCode) + "'>Click Aqui Para Activar tu Cuenta.</a>";
   body += "<br /><br />Gracias, Team LinknRed";
   body += "<br /><br />Por Favor, No Responda este Correo.";
   mm.Body = body;
   mm.IsBodyHtml = true;
   SmtpClient smtp = new SmtpClient();//smtpout.secureserver.net
   smtp.Host = "smtpout.secureserver.net";
   EnableSsl = true;
   NetworkCredential NetworkCred = new NetworkCredential("soporte@linknred.com", "********");
   smtp.UseDefaultCredentials = true;
   smtp.Credentials = NetworkCred;
   smtp.Port = 465;//465
   smtp.Send(mm);
}