Check if user has intruded a real email address. Return true (real email) or false (fake email).
Arguments: Element from a form.
From book Dom Scripting by Jeremy Keith

function isEmail(field) {
  if (field.value.indexOf("@") == -1 || field.value.indexOf(".") == -1) {
    return false;
  } else {
    return true;
  }
}