mailer symfony 4不工作

swiftmailer:
    transport: smtp
    host: smtp.example.com
    port: 587
    encryption: tls
    username: your_username
    password: your_password
    spool:
        type: file
        path: '%kernel.project_dir%/var/spool'

use Symfony\Component\Mailer\MailerInterface;
use Symfony\Component\Mime\Email;

class MyController
{
    private $mailer;

    public function __construct(MailerInterface $mailer)
    {
        $this->mailer = $mailer;
    }

    public function sendEmail()
    {
        $email = (new Email())
            ->from('sender@example.com')
            ->to('recipient@example.com')
            ->subject('Hello')
            ->text('This is a test email.');

        $this->mailer->send($email);

        return new Response('Email sent.');
    }
}