SwiftMailer does not send mail, why?

SwiftMail does not send my email, while mail() does work. Same as here.

I added the EchoLogger but it does not print anything.

$message = Swift_Message::newInstance();
 $message->setSubject('Test');
 $message->setTo( $email );
 $message->setFrom(ABSENDER);
 $message->setBody($nl_text);
 
 $transport = Swift_MailTransport::newInstance();
 $mailer = Swift_Mailer::newInstance($transport);
 
 $logger = new Swift_Plugins_Loggers_EchoLogger();
 $mailer->registerPlugin(new Swift_Plugins_LoggerPlugin($logger));
 
 $result = $mailer->send($message, $failures);
 var_dump($failures);
 

The email is returned in $failures but why?

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