public function convert(Email $email): \Swift_Message
    {
        $message = $this->createInstance()->setSubject($email->getSubject());
        $message->setBoundary($boundary = \md5(\uniqid()));

        $this->addAddresses($email, $message);
        $this->addParts($email, $message, $boundary);
        $this->addHeaders($email, $message);
        $this->addAttachments($email, $message);

        if ($contentType = $email->getContentType()) {
            $message->setContentType($contentType);
        }

        return $message;
    }