WordPress at a glance
function is not described

fix_phpmailer_messageid() WP 3.0.0

Correct From host on outgoing mail to match the site domain

No Hooks.

Return

Nothing (null).

Usage

fix_phpmailer_messageid( $phpmailer );
$phpmailer(PHPMailer) (required)
The PHPMailer instance (passed by reference).

Code of fix phpmailer messageid: wp-includes/ms-functions.php VER 5.1.1

<?php
function fix_phpmailer_messageid( $phpmailer ) {
	$phpmailer->Hostname = get_network()->domain;
}