protected function rcptTo()
    {
        $to = array_merge(
            $this->message->getTo(),
            $this->message->getCc(),
            $this->message->getBcc()
        );
        foreach ($to as $toEmail=>$_) {
            $in = "RCPT TO:<" . $toEmail . ">" . $this->CRLF;
            $code = $this->pushStack($in);
            if ($code !== '250') {
                throw new CodeException('250', $code, array_pop($this->resultStack));
            }
        }
        return $this;
    }