private function buildAttachmentBody($attachment)
    {
        return sprintf('%s%s%s%s%s%s',
            '--MIME_boundary'.self::CRLF,
            'Content-Type: '.$attachment['mimeType'].self::CRLF,
            'Content-Transfer-Encoding: binary'.self::CRLF,
            'Content-ID: <attachment.bin@devbay.net>'.self::CRLF.self::CRLF,
            $attachment['data'].self::CRLF,
            '--MIME_boundary--'
        );
    }