The memory spool is flushed on the kernel.terminate event that occurs after a response was sent. Seems that this event is not called on commands. Seen on http://stackoverflow.com/questions/13122096/unable-to-send-e-mail-from-within-custom-symfony2-command-but-can-from-elsewhere

email_in_command.php

$this
    ->getContainer()
    ->get('mailer')
    ->getTransport()
    ->getSpool()
    ->flushQueue(
        $this
          ->getContainer()
          ->get('swiftmailer.transport.real')
    );