public function sendAccountActivationLink(Array $data, Swift_Mailer $mailer)
{
    $message = (new Swift_Message('Hello Email'))
        ->setFrom('myemail@gmail.com')
        ->setTo($data['email'])
        ->setBody(
            $this->renderView(
                'mails/registration.html.twig',
                ['data' => $data]
            ),
            'text/html'
        )
    ;

    $mailer->send($message);
}

$this->get('mailer')->send($message);

swiftmailer:
    url: '%env(MAILER_URL)%'
    spool: { type: 'memory' }
    transport: smtp
    username: myemail@gmail.com
    password: password
    host: localhost
    port: false

MAILER_URL=gmail://myemail@gmail.com:password@localhost