src/Viserio/Component/Mail/Transport/PostmarkTransport.php
protected function getMessageId(?ResponseInterface $response): string
    {
        if ($response === null) {
            return '';
        }

        $content = \json_decode($response->getBody()->getContents(), true);

        return $content['MessageID'];
    }
protected function getMessageId(?ResponseInterface $response): string
    {
        if ($response === null) {
            return '';
        }

        $content = \json_decode($response->getBody()->getContents(), true);

        return $content['MessageID'];
    }
Get the message ID from the response.

@param null|\Psr\Http\Message\ResponseInterface $response

@return string