How do I override the Message-ID header of wp_mail function?

Question

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.

Answer 1

You can filter the $phpmailer object. Something like this should do the trick (not tested):

add_action( 'phpmailer_init', 'wpse_52555_msg_id' );

function wpse_52555_msg_id( &$phpmailer )
{
    $msg_id = get_post_meta( get_the_ID(), 'messageID', TRUE );
    '' !== $msg_id and $phpmailer->MessageID = $msg_id . '@test.com';
}

Answer 2

Anyone looking to just update the hostname inside the messageID, which may be needed for Nginx setup. Wordpress documentation.

add_action('phpmailer_init', 'sender_message_id');

function sender_message_id(&$phpmailer) {
  $phpmailer->Hostname = 'domain.com';
}

Answer 3

I think I found the solution to the initial problem.

You must, of course, use the phpmailer_init action to change the message-ID property of object phpmailer (used by wp_mail function). Thanks to Fuxia for the answer.

It is also necessary to respect message-ID format in accordance with the rfc5322 section-3.6.4 otherwise your new message-ID will be rejected (consult here the source file of phpmailer where the message-ID is setted).

Here is the correct format of a message-ID : <leftpart@rightpart> including brackets

Here is the code tested that works:

add_action('phpmailer_init', 'set_message_id');

function set_message_id(&$phpmailer) {
  $leftpart = 'mycustominfo-'.uniqid();
  $domain = 'mydomain.com';
  $messageID = '<'.$leftpart.'@'.$domain.'>';
  $phpmailer->MessageID = $messageID;
}

I hope it will help