PHPMailer\PHPMailer

PHPMailer::getLastMessageID │ public │ WP 1.0

Return the Message-ID header of the last email. Technically this is the value from the last time the headers were created, but it's also the message ID of the last sent message except in pathological cases.

Method of the class: PHPMailer{}

No Hooks.

Returns

String.

Usage

$PHPMailer = new PHPMailer();
$PHPMailer->getLastMessageID();

PHPMailer::getLastMessageID() code WP 6.8.1

wp-includes/PHPMailer/PHPMailer.php 
public function getLastMessageID()
{
    return $this->lastMessageID;
}