我是用Gandi.net server的，我原本是用XAMPP主機測試網頁都一切正常，但我把網頁搬到Gandi.net server去，就得到訊息：SMTP Error: Could not authenticate.PHPMailer就不能運作了。有人知道解決方法嗎？
以下是我目前的程式碼，是有那裡出錯了嗎？

<?php
use PHPMailer\PHPMailer\PHPMailer;

require_once 'phpmailer/Exception.php';
require_once 'phpmailer/PHPMailer.php';
require_once 'phpmailer/SMTP.php';

$mail = new PHPMailer(true);

$alert = '';

if(isset($_POST['submit'])){
  $name = $_POST['name'];
  $email = $_POST['email'];
  $phone = $_POST['phone'];
  $booking = $_POST['booking'];
  $bookDate = $_POST['bookDate'];

  try{
    $mail->isSMTP();
    $mail->Host = 'smtp.gmail.com';
    $mail->SMTPAuth = true;
    $mail->Username = 'XXXXX@gmail.com';
    $mail->Password = 'XXXXXX'; // Gmail address Password
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
    $mail->Port = '587';

    $mail->setFrom('XXXXX@gmail.com');
    $mail->addAddress('XXXXX@gmail.com');  

    $mail->CharSet = 'UTF-8';

    $mail->isHTML(true);
    $mail->Subject = "客戶預約($name)";
    $mail->Body = "<h3>姓名 : $name <br>電郵: $email <br>電話 : $phone <br>預約服務 : $booking <br>預約時間 : $bookDate </h3>";

    $mail->send();
    $alert = '<div class="alert-success">
                 <span>Message Sent! Thank you for contacting us.</span>
                </div>';
  } catch (Exception $e){
    $alert = '<div class="alert-error">
                <span>'.$e->getMessage().'</span>
              </div>';
  }
}
?>