protected function mergeTransportResult(ResultContract $transportResult, ResultContract $mailerResult)
    {
        if (!$mailerResult instanceof SendResult) {
            throw new InvalidArgumentException('mergeTransportResult can only merge Ems\Mail\SendResult');
        }

        $mailerResult->increment(count($transportResult));

        foreach ($transportResult->failures() as $i => $failed) {
            $mailerResult->addFailedRecipient($failed, $transportResult->error($i));
        }
    }