protected function _getPos($id)
    {
        if (!isset($this->_positions[$id - 1])) {
            /**
             * @see Zend_Mail_Storage_Exception
             */
            // require_once 'Zend/Mail/Storage/Exception.php';
            throw new Zend_Mail_Storage_Exception('id does not exist');
        }

        return $this->_positions[$id - 1];
    }