function checkname(form){
  if (form.email.value== "") {
    alert(" Introduzca su E-Mail, por favor");
    form.email.value = "";
    form.email.focus();
    return false;
  } 
  if (form.email.value.indexOf('@',0) == -1){
    alert("Direccion E-Mail no valida");
    form.email.value = "";
    form.email.focus();
    return false;
  }
  if (form.email.value.indexOf('.',0) == -1){
    alert("Direccion E-Mail no valida");
    form.email.value = "";
    form.email.focus();
    return false;
  }
  if (form.comentario.value== "") {
    alert(" Introduzca su comentario, por favor");
    form.comentario.value = "";
    form.comentario.focus();
    return false;
  } 
}