This is the class where the service mailer should be injected:

namespace Gremo\SkebbyBundle\Transport;

use Swift_Mailer;
use Gremo\SkebbyBundle\Message\AbstractSkebbyMessage;

class MailerTransport extends AbstractSkebbyTransport
{
    /**
     * @var \Swift_Mailer
     */
    private $mailer;

    public function __construct(Swift_Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    /**
     * @param \Gremo\SkebbyBundle\Message\AbstractSkebbyMessage $message
     * @return void
     */
    public function executeTransport(AbstractSkebbyMessage $message)
    {
        /* ... */
    }
}

Service id is gremo_skebby.transport.mailer, placed inside mailer.xml file:

<parameters>
    <parameter key="gremo_skebby.converter.swift_message.class">
        Gremo\SkebbyBundle\Converter\SwiftMessageConverter
    </parameter>
    <parameter key="gremo_skebby.transport.mailer.class">
        Gremo\SkebbyBundle\Transport\MailerTransport
    </parameter>
</parameters>

<services>
    <service id="gremo_skebby.converter.swift_message" 
        class="%gremo_skebby.converter.swift_message.class%" public="false" />

    <service id="gremo_skebby.transport.mailer" 
        class="%gremo_skebby.transport.mailer.class%" public="false"
        parent="gremo_skebby.tranport.abstract_transport">
        <argument type="service" id="mailer" />
    </service>
</services>

When i try to inject the gremo_skebby.transport.mailer into another service (an helper) i get:

Symfony\Component\DependencyInjection\Exception\InvalidArgumentException : The service definition "mailer" does not exist.

That is strange, because command php app/console container:debug shows that the mailer service actually exists:

mailer container Swift_Mailer

I'm loading mailer.xml file dynamically by the extension:

if(in_array($configs['transport'], array('http', 'rest', 'mailer'))) {
    $loader->load('transport.xml');
    $loader->load($configs['transport'] . '.xml');

    $container->setAlias('gremo_skebby.transport',
        'gremo_skebby.transport.' . $configs['transport']);
}

$loader->load('skebby.xml');

... and gremo_skebby.transport service is injected into gremo_skebby.skebby service (skebby.xml):

<parameters>
    <parameter key="gremo_skebby.skebby.class">
        Gremo\SkebbyBundle\Skebby
    </parameter>
</parameters>

<services>
    <service id="gremo_skebby.skebby" class="%gremo_skebby.skebby.class%">
        <argument type="service" id="gremo_skebby.transport" />
    </service>
</services>

A quick test is giving me the same InvalidArgumentException:

public function testSkebbyWithMailerTransport()
{
    $loader    = new GremoSkebbyExtension();
    $container = new ContainerBuilder();
    $config    = $this->getEmptyConfiguration();

    $loader->load(array($config), $container);

    $this->assertTrue($container->hasDefinition('gremo_skebby.transport.mailer'));
    $this->assertTrue($container->hasDefinition('gremo_skebby.skebby'));

    $this->assertInstanceOf('Gremo\SkebbyBundle\Skebby',
        $container->get('gremo_skebby.skebby'));
}