$admin undefined

easy fix
in .module line 506 $admin variable is present.
If you are not an admin, this will generate a warning.
if $admin is declared as FALSE on line 486 there will be no problem

function mailing_list_subscription_form_submit($form, &$form_state) {
  $is_edit = FALSE;
  $admin = FALSE;
  if (user_access('administer mailing lists')) {
    $admin = TRUE;
    $is_edit = !empty($form_state['values']['eid']);
  }

  if ($form_state['values']['mlid'] == null) {
    drupal_set_message(t('Unable to save mailing list subscription: no mailing list specified.'), 'error');
    return;
  }

  $mle = new StdClass();
  $mle->mlid = $form_state['values']['mlid'];
  $mle->mail = $form_state['values']['mail'];
  $mle->name = $form_state['values']['name'];
  if ($is_edit) {
    $mle->eid = $form_state['values']['eid'];
  }

  if (mailing_list_email_save($mle)) {
    if ($admin) {
      if ($is_edit) {
        drupal_set_message('<span class="mailing-list-subscription-message">' . t(variable_get('mailing_list_message_admin_email_edited', "%name <%mail> updated."), array('%name' => $form_state['values']['name'], '%mail' => $form_state['values']['mail'])). '</span>');
        return;
      }
      global $user;
      drupal_set_message('<span class="mailing-list-subscription-message">' . t(variable_get('mailing_list_message_admin_email_added', "Submission confirmed: %admin added %name <%mail> to list %list."), array('%name' => $form_state['values']['name'], '%mail' => $form_state['values']['mail'], '%admin' => $user->name, '%list' => $form_state['values']['ml_name'])). '</span>');
      return;
    }
    drupal_set_message('<span class="mailing-list-subscription-message">' . t(variable_get('mailing_list_subscription_message', MAILING_LIST_MESSAGE_SUBSCRIPTION), array('%name' => $form_state['values']['name'], '%mail' => $form_state['values']['mail'])). '</span>');
    watchdog('mailing_list', 'Mailing list: %name added via subscription form.', array('%name' => $form_state['values']['mail']));
  }
  else {
    drupal_set_message(t('Failed to subscribe to mailing list %name.', array('%name' => $form_state['values']['ml_name'])), 'error');
  }
}