public function getMailHeaders()
    {
        $headers = [];

        $html = $this->getHtml();
        $text = $this->getText();
        $subject = $this->getSubject();

        if ($html != "" || count($this->attachments) > 0) {
            $mime = new MimeDocument("multipart/alternative", crc32($html) . crc32($text) . crc32($subject));
            $headers = $mime->getHeaders();
        } else {
            $headers["Content-Type"] = "text/plain; charset=utf-8";
        }

        $headers["From"] = (string)$this->getSender();
        $headers["Subject"] = $subject;

        return $headers;
    }