$message = Swift_Message::newInstance()
    ->setSubject('Registo efetuado com sucesso')
    ->setFrom(array('noreply@xxxx.xxxx' => 'XXXXXX'))
    ->setTo(array($to))
    ->setBody($message, 'text/html')
;

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

swiftmailer:
    transport: smtp
    host: <SERVIDOR_SMTP>
    port: 587
    encryption: tls
    username:  <USER>
    password:  <PASS>