DKIM failes when I add an AltBody

[julesbl]

Problem description

phpMailer 6.8.1, php v7.4
If I send the test email with just a HTML body it works fine, if I send with an AltBody as well I get a DKIM error

Code to reproduce

	$this->emailer = new PHPMailer(true);
	if(ENVIRONMENT == 'development'){
		$this->emailer->SMTPDebug = SMTP::DEBUG_SERVER;
	}
	$this->emailer->isSMTP();
	$this->emailer->Host = $this->config->smtp_server;
	$this->emailer->SMTPAuth = true;
	$this->emailer->Username = $this->config->smtp_username;
	$this->emailer->Password = $this->config->smtp_password;
	$this->emailer->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;
	$this->emailer->Port = $this->config->smtp_ssl_port;
	$this->emailer->isHTML(true);

	$this->twig = service('twig');

	$content = [
		'title' => 'Test email',
		'company_name' => 'Routastic',
	];
	$email = $this->twig->render('emails/test.twig.html', $content);
	$text = strip_a($email);
	try {
		$this->emailer->setFrom($this->config->admin_email, 'Routastic admin');
		$this->emailer->addAddress($email);
		$this->emailer->addReplyTo($this->config->admin_email, 'Routastic admin');

		$this->emailer->Subject = 'Test Email';
		$this->emailer->Body = $email;
		$this->emailer->AltBody = $text;
		$this->emailer->send();
	}catch (\Exception $e){
		trigger_error('Failed to send test email: '.$e->getMessage(), E_USER_WARNING);
		return new Error('Failed to send test email');
	}
	return true;

Debug output

[XL-2000]

To make use of DKIM, you need to configure PHP to do so.
Normally done by setting all needed settings $this->emailer->DKIM_*
(where * are the actual settings like domain, selector, identity, private_key, etc)

Since your code does not set any DKIM instructions, it is logical DKIM is unsigned.

I can not believe it is true that DKIM is working for you if you only set the HTML Body. Maybe your SPAM checker does not raise an error, but it will absolutely not be DKIM signed given your current code

[julesbl]

Its configured as part of the mail sending service and is beyond my remit.
The strange thing is https://dkimvalidator.com thinks its ok but https://aboutmy.email thinks there is a problem

[XL-2000]

1. To be clear; are you using the PHPMailer signing functionality? And if so, can you share that code, just in case?
2. What does strip_a(...) do? It seems you are pushing a full-blown HTML rendered mail template into a plain-text body-part

Also one thing I notice; you are using:

$this->emailer->Body = $email;
$this->emailer->AltBody = $text;

What happens if you use:

$this->emailer->msgHTML($email);
$this->emailer->AltBody = $text;

[julesbl]

OK, tested this with several email testers and it only errors on one of them so I've reported the bug in their system to them, don't think there is anything wrong with phpMail.
Thanks for the help.
Jules