private function storeHeader($name, Swift_Mime_Header $header, $offset = null)
    {
        if (!isset($this->headers[strtolower($name)])) {
            $this->headers[strtolower($name)] = [];
        }
        if (!isset($offset)) {
            $this->headers[strtolower($name)][] = $header;
        } else {
            $this->headers[strtolower($name)][$offset] = $header;
        }
    }