<?php
$mailer = Swift_Mailer::newInstance(
  Swift_SmtpTransport::newInstance('smtp.example.org', 25)
);

// NOTIFY=NEVER
$mailer->registerPlugin(new DsnPlugin([new NotifyHandler('never')]));

// --- OR ---

// NOTIFY=SUCCESS,FAILURE
$mailer->registerPlugin(new DsnPlugin([new NotifyHandler('success,failure')]));