wp-includes/PHPMailer/PHPMailer.php:1478Return the Message-ID header of the last email.
string public function getLastMessageID() { return $this->lastMessageID; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.