Pourquoi PHP mail retourne false ?

namespace PHPMailer\PHPMailer;

$mail = new PHPMailer\PHPMailer\PHPMailer(true);

use PHPMailer\PHPMailer\PHPMailer as zorglub;
$mail = new zorglub(true);

use PHPMailer\PHPMailer\PHPMailer;

use PHPMailer\PHPMailer\PHPMailer as PHPMailer;

//Enable SMTP debugging
//SMTP::DEBUG_OFF = off (for production use)
//SMTP::DEBUG_CLIENT = client messages
//SMTP::DEBUG_SERVER = client and server messages
$mail->SMTPDebug = SMTP::DEBUG_SERVER;

//Send the message, check for errors
if (!$mail->send()) {
    echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
    echo 'Message sent!';
}

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;

include 'PHPMailer/src/PHPMailer.php';
include 'PHPMailer/src/SMTP.php';
include 'PHPMailer/src/Exception.php';

$mail = new PHPMailer(true);

$Mail = $_GET["MailTo"];
$Subject = $_GET["Subject"];
$Body = $_GET["Body"];

try {
    $mail->SMTPDebug = SMTP::DEBUG_SERVER;                      //Enable verbose debug output

// Gmail Server settings
    $mail->isSMTP();                                            //Send using SMTP
    $mail->Host       = 'smtp.gmail.com';                     //Set the SMTP server to send through
    $mail->SMTPAuth   = true;                                   //Enable SMTP authentication
    $mail->Username   = 'Mon adresse@gmail.com';                     //SMTP username
    $mail->Password   = 'Mon Mdp gmail';                               //SMTP password
    $mail->setFrom('Mon adresse@gmail.com', 'RVReefTools');

    $mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;            //Enable implicit TLS encryption
    $mail->Port       = 465;                                    //TCP port to connect to; use 587 if you have set `SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS`

    $mail->addAddress($Mail);     //Add a recipient
    $mail->isHTML(true);                                  //Set email format to HTML
    $mail->Subject = $Subject;
    $mail->Body    = "<HTML><BODY>".$Body."</body></html>";

    $mail->send();
    echo 'Sent';
} catch (Exception $e) {
    echo "Mailer Error : {$mail->ErrorInfo}";
}

isSMTP()

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