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Correct From: host on outgoing email to match the site domain. It is just a wrapper function for:
$phpmailer->Hostname = $current_site->domain;
<?php fix_phpmailer_messageid( $phpmailer ) ?>
The PHPMailer object is modified directly, no value is returned.
fix_phpmailer_messageid( $phpmailer ); echo $phpmailer->Hostname;
fix_phpmailer_messageid() is located in wp-includes/ms-functions.php
wp-includes/class-phpmailer.php - the PHPMailer class file within WordPress core.