Php邮件工作正常，但以纯文本格式获得输出

require 'PHPMailer/PHPMailerAutoload.php';

$mail = new PHPMailer;
$mail->isSMTP();
$mail->Host = 'smtp.example.com';
$mail->SMTPAuth = true;
$mail->Username = 'your-email@example.com';
$mail->Password = 'your-email-password';
$mail->SMTPSecure = 'tls';
$mail->Port = 587;

$mail->setFrom('your-email@example.com', 'Your Name');
$mail->addAddress('recipient@example.com', 'Recipient Name');
$mail->isHTML(true);

$mail->Subject = 'Test Email';
$mail->Body = '<h1>This is a test email</h1><p>Hello, world!</p>';

if ($mail->send()) {
    echo 'Email sent successfully';
} else {
    echo 'Email could not be sent';
}

$to = 'recipient@example.com';
$subject = 'Test Email';
$message = 'This is a test email. Hello, world!';

$headers = 'From: your-email@example.com' . "\r\n" .
    'Reply-To: your-email@example.com' . "\r\n" .
    'Content-Type: text/plain; charset=UTF-8' . "\r\n" .
    'X-Mailer: PHP/' . phpversion();

if (mail($to, $subject, $message, $headers)) {
    echo 'Email sent successfully';
} else {
    echo 'Email could not be sent';
}