https://johnlinhart.com/blog/uh-oh-mautic-upgrade-was-not-successful
Add Mailto to the List-Unsubscribe Header:
Go to -> /app/bundles/EmailBundle/Helper/MailHelper.php:
Aproximately on line 1484
/**
* Generate and insert List-Unsubscribe header.
*/
private function addUnsubscribeHeader()
{
if (isset($this->idHash)) {
$unsubscribeLink = $this->factory->getRouter()->generate('mautic_email_unsubscribe', ['idHash' => $this->idHash], true);
$this->headers['List-Unsubscribe'] = "<unsubscribe@[DOMAIN]>,<$unsubscribeLink>";
}
}