public function getMultipartHelper()
    {
        if ($this->multipartHelper === null) {
            $this->multipartHelper = new MultipartHelper(
                $this->partFactoryService->getMimePartFactory(),
                $this->partFactoryService->getUUEncodedPartFactory(),
                $this->partBuilderFactory,
                $this->getGenericHelper()
            );
        }
        return $this->multipartHelper;
    }