public function setHeaderEncoding($encoding)
    {
        $allowed = array(
            Zend_Mime::ENCODING_BASE64,
            Zend_Mime::ENCODING_QUOTEDPRINTABLE
        );
        if (!in_array($encoding, $allowed)) {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Invalid encoding "' . $encoding . '"');
        }
        $this->_headerEncoding = $encoding;

        return $this;
    }