Add validation to an existing form (such as a webform content type)

/* email list web form validation */
function custom_form_alter(&$form, $form_state, $form_id) {
 
  if ($form_id == 'webform_client_form_125') {
    $form['webform-client-form-125']['email']['#element_validate'] = array('_custom_email_list_email_validate');
  }
}
 
function _custom_email_list_email_validate($form, &$form_state) {
 
$values = $form_state['values']['submitted'];
 
require_once('is_email.inc.php');
 
  if (!is_email($values['email'])) {
    form_set_error('email', t('Please enter a valid email address.'));
  }
}