A change to libraries/vendor/phpmailer/phpmailer/class.phpmailer.php (method mailPassthru) introduced in #33772 breaks sending of mail if phpmailer is configured to do DKIM signing (and phpmailer is configured to use php's mail()). Need also php >= 8.0 or (presumably) a windows setup (have only tested on linux + php 8.0).

The relevant patch from #33772 in libraries/vendor/phpmailer/phpmailer/class.phpmailer.php:mailPassthru()

    // Joomla Backported concept from https://github.com/PHPMailer/PHPMailer/pull/2188
    if (PHP_VERSION_ID >= 80000 || stripos(PHP_OS, 'WIN') === 0)
    {
        $header = str_replace("\n", self::CRLF, $header);
    }

causes a subsequent failure if DKIM signing is done by phpmailer because phpmailer adds a "\r\n" before the dkim header, so the above patch results in this changing to "\r\r\n".
The subsequent call to mail() will fail.

Changing the str_replace line above to something like

        $header = preg_replace("/(?<!\r)\n/",self::CRLF,$header);

fixes the problem.

Steps to reproduce the issue

Need php mail to be the joomla mailer and either php >= 8.0 or (presumably) windows os:

• Set the joomla mailer object to do DKIM signing, eg in a system plugin onAfterInitialise():

    $mailer_clone = JFactory::getMailer(); 
    $mailer = JFactory::$mailer
    $mailer->DKIM_domain = $signing_domain // eg 'example.com';
    $mailer->DKIM_private = $path_to_dkim_private_key;
    $mailer->DKIM_selector = $dkim_selector; // eg 'mail';
    $mailer->DKIM_passphrase = '';
    $mailer->DKIM_identity = $site_email_address; // eg 'website@example.com';
  

• Any subsequent sending of mail by Joomla will fail.

Thanks, David