protected function getSectionMultipartAlternative()
    {
        $output = $this->boundaryDelimiterLine('multipart/alternative')
                . $this->contentInfoString('text/plain')
                . $this->getSectionTextPlain()
                . $this->boundaryDelimiterLine('multipart/alternative')
                . $this->contentInfoString('text/html')
                . $this->getSectionTextHtml()
                . $this->finalBoundaryDelimiterLine('multipart/alternative')
        ;

        return $output;
    }