$transporter = Swift_SmtpTransport::newInstance('myWebMail.com', 465, 'ssl')
        ->setUsername('user@test.com')
        ->setPassword('secret');
$mailer = Swift_Mailer::newInstance($transporter);

Swift_Mailer::__set_state(array(
   '_transport' =>
  Swift_SmtpTransport::__set_state(array(
     '_handlers' =>
    array (
      'AUTH' =>
      Swift_Transport_Esmtp_AuthHandler::__set_state(array(
         '_authenticators' =>
        array (
          0 =>
          Swift_Transport_Esmtp_Auth_CramMd5Authenticator::__set_state(array(
          )),
          1 =>
          Swift_Transport_Esmtp_Auth_LoginAuthenticator::__set_state(array(
          )),
          2 =>
          Swift_Transport_Esmtp_Auth_PlainAuthenticator::__set_state(array(
          )),
          3 =>
          Swift_Transport_Esmtp_Auth_NTLMAuthenticator::__set_state(array(
          )),
          4 =>
          Swift_Transport_Esmtp_Auth_XOAuth2Authenticator::__set_state(array(
          )),
        ),
         '_username' => 'user@test.com',
         '_password' => 'secret',
         '_auth_mode' => NULL,
         '_esmtpParams' =>
        array (
        ),
      )),
      ...

$mailer = $this->container->get('swiftmailer.mailer');

Swift_Mailer::__set_state(array(
   '_transport' =>
  Swift_Transport_EsmtpTransport::__set_state(array(
     '_handlers' =>
    array (
      'AUTH' =>
      Swift_Transport_Esmtp_AuthHandler::__set_state(array(
         '_authenticators' =>
        array (
          0 =>
          Swift_Transport_Esmtp_Auth_CramMd5Authenticator::__set_state(array(
          )),
          1 =>
          Swift_Transport_Esmtp_Auth_LoginAuthenticator::__set_state(array(
          )),
          2 =>
          Swift_Transport_Esmtp_Auth_PlainAuthenticator::__set_state(array(
          )),
        ),
         '_username' => 'user@test.com',
         '_password' => 'secret',
         '_auth_mode' => NULL,
         '_esmtpParams' =>
        array (
        ),
      )),
    ),

mailer_transport: smtp
mailer_host: myWebMail.com
mailer_port: 465
mailer_encryption: ssl
mailer_user: user@test.com
mailer_password: secret

swiftmailer:
    default_mailer: default_mailer
    mailers:
        default_mailer:
            transport: "%mailer_transport%"
            host:      "%mailer_host%"
            username:  "%mailer_user%"
            password:  "%mailer_password%"