我收到消息无法发送。邮件错误：SMTP错误：无法验证

Question

try{
    $stmt = $conn->prepare("INSERT INTO users (email, password, firstname, lastname, activate_code, created_on) VALUES (:email, :password, :firstname, :lastname, :code, :now)");
    $stmt->execute(['email'=>$email, 'password'=>$password, 'firstname'=>$firstname, 'lastname'=>$lastname, 'code'=>$code, 'now'=>$now]);
    $userid = $conn->lastInsertId();

    $message = "
            <h2>Thank you for Registering.</h2>
            <p>Your Account:</p>
            <p>Email: ".$email."</p>
            <p>Password: ".$_POST['password']."</p>
            <p>Please click the link below to activate your account.</p>
        <a href='http://localhost/ecommerce/activate.php?code=".$code."&user=".$userid."'>Activate Account</a>
    ";

    //Load phpmailer
    require 'vendor/autoload.php';

    $mail = new PHPMailer(true);                             
    try {
        //Server settings
        $mail->isSMTP();                                     
        $mail->Host = 'smtp.gmail.com';                      
        $mail->SMTPAuth = true;                               
        $mail->Username = 'iam4759@gmail.com';     
        $mail->Password = 'mysourcepass';                    
        $mail->SMTPOptions = array(
            'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false,
            'allow_self_signed' => true
            )
        );                         
        $mail->SMTPSecure = 'ssl';                           
        $mail->Port = 465;                                   

        $mail->setFrom('iam4759@gmail.com');

        //Recipients
        $mail->addAddress($email);              
        $mail->addReplyTo('iam4759@gmail.com');

        //Content
        $mail->isHTML(true);                                  
        $mail->Subject = 'Pet centre';
        $mail->Body    = $message;

        $mail->send();

        unset($_SESSION['firstname']);
        unset($_SESSION['lastname']);
        unset($_SESSION['email']);

        $_SESSION['success'] = 'Account created. Check your email to activate.';
        header('location: signup.php');

    } 
    catch (Exception $e) {
        $_SESSION['error'] = 'Message could not be sent. Mailer Error: '.$mail->ErrorInfo;
        header('location: signup.php');
    }