SwiftMailer silently ignores errors when messages fail delivery

When run from inside controller and when in-memory spooling is configured via spool: { type: memory } swiftmailer seems to work like this:

• whenever from within controller mailer->send($message) is called -> save message in memory
• when controller has finished work and symfony Kernel is about to shutdown (somwehere in event kernel.terminate or smth) - check messages saved in memory and _actually submit them to SMTP-server

However this last step seems to silently ignore any errors which may be thrown when peforming submitting of the message to the SMTP server.

I discovered that errors are silently swallowed when I was setting up SMTP from Amazon SES, and have made wrong configuration:

mailer_transport: smtp
# For illustration I put WRONG port 9999, which means that this should trigger
# error (correct port would be 587)
mailer_port: 9999
mailer_encryption: tls
mailer_host: email-smtp.us-east-1.amazonaws.com
mailer_user: SES_USER_KEY
mailer_password: SES_USER_SECRET 

Now, if I attempt to send email using wrong configuration from a symfony Command, just as expected I get Swift_TransportException and error is NOT silently ignored. (From my observations it seems that symfony commands do NOT use memory-spooling and attempt to send messages immediately)

Below is sample of the command (so you're sure I am doing it right).

protected function execute(InputInterface $input, OutputInterface $output) {
     $email = $input->getArgument('email');
     $content = $this->getHelper('dialog')->ask($output, 'Please input content:');

     $emsg = \Swift_Message::newInstance();
     $emsg->setTo($email);
     $emsg->setFrom('d@my-ses-verified-domain.com');
     $emsg->setSubject('This is subject');
     $emsg->setBody($content);

     $this->getContainer()->get('mailer')->send($emsg);
}

And here's command output when the exception Swift_TransportException is thrown:

ubuntu@localhost:~/my-app$ console acme:email:send existing@email.com
We are going to send email to :existing@email.com

Please input content:asdf adf


[Swift_TransportException]                                                                                    
Connection could not be established with host email-smtp.us-east-1.amazonaws.com [Connection timed out #110]  

Hovewer if I attempt to send email from controller, then I see no error messages. Basically this means that in case there's an error (misconfiguration or network error or SMTP server down), all the emails I sent will silently disappear without any trace (no exception thrown, no error loggged in dev.log neither in prod.log).

How can I force Swiftmailer to leave trace of failed delivery attempt?