AcMailer\Service\Factory\MailServiceAbstractFactory::createTransport PHP Method

    protected function createTransport(ContainerInterface $container)
    {
        $adapter = $this->mailOptions->getMailAdapter();
        // A transport instance can be returned as is
        if ($adapter instanceof TransportInterface) {
            return $this->setupTransportConfig($adapter);
        }
        // Check if the adapter is a service
        if (is_string($adapter) && $container->has($adapter)) {
            /** @var TransportInterface $transport */
            $transport = $container->get($adapter);
            if ($transport instanceof TransportInterface) {
                return $this->setupTransportConfig($transport);
            } else {
                throw new InvalidArgumentException('Provided mail_adapter service does not return a "Zend\\Mail\\Transport\\TransportInterface" instance');
            }
        }
        // Check if the adapter is one of Zend's default adapters
        if (is_string($adapter) && is_subclass_of($adapter, 'Zend\\Mail\\Transport\\TransportInterface')) {
            return $this->setupTransportConfig(new $adapter());
        }
        // The adapter is not valid. Throw an exception
        throw new InvalidArgumentException(sprintf('mail_adapter must be an instance of "Zend\\Mail\\Transport\\TransportInterface" or string, "%s" provided', is_object($adapter) ? get_class($adapter) : gettype($adapter)));
    }