public function setReturnPath($email)
    {
        if ($this->_returnPath === null) {
            $email = $this->_filterEmail($email);
            $this->_returnPath = $email;
            $this->_storeHeader('Return-Path', $email, false);
        } else {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Return-Path Header set twice');
        }
        return $this;
    }