Phpmailer和联系表格，成功消息，但没有结果

Here my php code for contact form using PhpMailer.

<?php 
    use PHPMailer\PHPMailer\PHPMailer;
    include_once "PHPMailer/PHPMailer.php";
    include_once "PHPMailer/Exception.php";
    include_once "PHPMailer/SMTP.php";

    if (isset($_POST['submit'])) {
        $subject = $_POST['subject'];
        $email = $_POST['email'];
        $message = $_POST['message'];
        echo $subject . "<br/>";
        echo $email . "<br/>";
        echo $message . "<br/>";
        $mail = new PHPMailer();

        $mail->addAddress('example@gmail.com');
        $mail->setFrom($email);
        $mail->Subject = $subject;
        $mail->isHTML(true);
        $mail->Body = $message;

        if ($mail->send())
            echo "Your email has been sent, thank you!";
        else
            echo "Please try again!";

    }
?>

It shows "Your email has been sent, thank you", but does not send anyting. How can solve it?