I am trying to attach an email directly to another another email.

Here is the source

        $mailer = new PHPMailer();
        $message = '
Some message body text<>';
        $mailer->Subject = 'Email subject';
        $mailer->addAddress('test@test.com');
        $mailer->Body = $message;
        $mailer->FromName = $_SERVER['SERVER_NAME'];
        $mailer->Sender = $mailer->From = 'sender@test.com';
        $mailer->isHTML(true);

        // Also attach the raw email as .eml file for better handling/training purposes
        $attachmentMail = new PHPMailer();
        $attachmentMail->FromName = $violationFrom;
        $attachmentMail->addAddress('test@test.org');
        $attachmentMail->Subject = $violationSubject;
        $attachmentMail->isHTML(true);
        $attachmentMail->Body    = $violationBody;

        // Prepare and get raw MIME source
        if (!$attachmentMail->preSend()) {
            echo "preSend failed: " . $attachmentMail->ErrorInfo; die();
        }        
        $rawMime = $attachmentMail->getSentMIMEMessage();

        // Attach the raw MIME as an .eml file
        $mailer->addStringAttachment($rawMime, 'raw-email.eml',  PHPMailer::ENCODING_BASE64, 'message/rfc822');

        // Send the alert email
        $mailer->send();

I get no error when sending the email. In the email I receive there is an attachment called raw-email.eml which also has the proper size. However, no matter the email client I use I can neither download the attachment, nor open it. Any hint why?