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.

{} It's a method of the class: PHPMailer{}

No Hooks.

Return

String.

Usage

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

Code of PHPMailer::getLastMessageID() WP 6.0

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