I have a custom notification function for our comments editor, who prefers to have all comments from one article threaded together in her email client. To achieve this, I'm creating a custom message-ID for the first comment on an article, then setting that as the In-Reply-To for future comment notifications.
This is working to some extent - I can see the additional headers in the mail client - however, the first message is being created with TWO Message-IDs. In other words, the one I passed into the headers is NOT overriding the one Wordpress is autogenerating. Therefore, the threading doesn't work.
Is this a bug with Wordpress? I don't want to resort to hunting down the actual WP_mail function and editing core code; and I'm not sure that would even work. Is this something more fundamental with PHP Mail function that I can't change perhaps?
$messageIDtoCreate = $post->ID.".".time(); // post ID, and current timestamp add_post_meta( $post->ID, 'messageID', $messageIDtoCreate); // add to the email headers $message_headers .= "Message-ID: <".$messageIDtoCreate."@test.com>\n";
Thanks in advance.