Flushing the email memory spool in a symfony command

flush-spool.php

<?php

$mailer = $this->getContainer()->get('mailer');

$message = new \Swift_Message();

$message->setBody('');
$message->setSubject('');
$message->setTo('');
$message->setFrom('');

$mailer->send($message);

$spool = $mailer->getTransport()->getSpool();
// flush the spool if we are in developer mode
if ($spool instanceof \Swift_MemorySpool) {
    $transport = $this->getContainer()->get('swiftmailer.transport.real');
    $spool->flushQueue($transport);
}