public function rcpt($to)
    {
        if ($this->_mail !== true) {
            /**
             * @see Zend_Mail_Protocol_Exception
             */
            // require_once 'Zend/Mail/Protocol/Exception.php';
            throw new Zend_Mail_Protocol_Exception('No sender reverse path has been supplied');
        }

        // Set rcpt to true, as per 4.1.1.3 of RFC 2821
        $this->_send('RCPT TO:<' . $to . '>');
        $this->_expect(array(250, 251), 300); // Timeout set for 5 minutes as per RFC 2821 4.5.3.2
        $this->_rcpt = true;
    }