public function setSubject($subject)
    {
        if ($this->_subject === null) {
            $subject = $this->_filterOther($subject);
            $this->_subject = $this->_encodeHeader($subject);
            $this->_storeHeader('Subject', $this->_subject);
        } else {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Subject set twice');
        }
        return $this;
    }