public function setHTMLMessage($message)
    {
        if ($message === '') {
            throw new \InvalidArgumentException('$message must not be empty.', 1331488845);
        }

        if ($this->hasCssFile()) {
            $this->loadEmogrifierClass();
            $emogrifier = new Emogrifier($message, $this->getCssFile());
            $messageToStore = $emogrifier->emogrify();
        } else {
            $messageToStore = $message;
        }

        $this->setAsString('html_message', $messageToStore);
    }