Sending Mails using swift mailer in Symfony 4

Question

I am trying to sent emails via swift mail using symfony 4. Code below is I tried so far. But email is not sending.

.env

MAILER_URL=smtp://localhost:25?encryption=&auth_mode=

controller

/**
     * @Route("/registration", name="new_registration")
     * Method({"GET","POST"})
     */

    public function newRegistration(Request $request, \Swift_Mailer $mailer)
    {

        if ($form->isSubmitted() && $form->isValid()){

            $message = (new \Swift_Message('Hello Mail'))
                ->setFrom('noreply@cccc.org')
                ->setTo('abcccc@gmail.com')
                ->setBody(
                    $this->renderView(
                        'main/registrationEmail.html.twig',
                        ['name' => $name]
                    ),
                    'text/html'
                );
            try {
                $mailer->send($message);
            } catch (\Swift_TransportException $exception) {
                echo $exception->getMessage();
            } catch (\Exception $exception) {
                echo $exception->getMessage();
            }

            return $this->render('main/registrationSuccess.html.twig');
        }
        return $this->render('main/registration.html.twig');
    }

There is not showing any error. But mail is not sending. Please help me on this.

UPDATE

I tried to send by console and using php bin/console swiftmailer:email:send. From that I got below error.

2019-03-14T11:40:32+01:00 [error] Exception occurred while flushing email queue: Connection could not be established with host localhost [No connection could be made because the target machine actively refused it.
 #10061]