////////////////////////////////////
function validaEmail(campo){ 
  s = campo.value;
  if (s == "") 
       return false;

  if (s.indexOf("@@") != -1)
       return false;

  if (s.indexOf("..") != -1)
       return false;

  if (s.indexOf(".@") != -1)
       return false;

  if (s.indexOf("@.") != -1)
       return false;

  var i = 1;
  var sLength = s.length;
  while ((i < sLength) && (s.charAt(i) != "@")){
        i++;
  }
   if ((i >= sLength) || (s.charAt(i) != "@")) 
       return false;
    else 
       i += 2;
    while ((i < sLength) && (s.charAt(i) != ".")){
       i++;
    }
    if ((i >= sLength - 1) || (s.charAt(i) != ".")) 
       return false;
    else 
       return true;
}