use drupal_write_record() to save log item

This would remove the need for an INSERT query, which is a small simplification of the code, but more importantly, it handles incrementing the serial ID field, so this code that's far too close to database internals could be removed completely:

  switch ($GLOBALS['db_type']) {
    case 'mysql':
    case 'mysqli':
      $new_mlid = NULL;
      break;
    case 'pgsql':
      $new_mlid = db_result(db_query("SELECT nextval('{mail_logger_mlid_seq}')"));
      break;
  }