Symfony messenger and mailer : how to add a binding_key?

I have a running Symfony 4.4 project with messenger and rabbitMQ. I have an async transport with 2 queues.

    transports:
        # https://symfony.com/doc/current/messenger.html#transport-configuration
      async:
        dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
        options:
          exchange:
            name: myexchange
            type: direct
          queues:
            email:
              binding_keys:
                - email
            extranet:
              binding_keys:
                  - extranet
        # failed: 'doctrine://default?queue_name=failed'
        # sync: 'sync://'

    routing:
        # Route your messages to the transports
        'App\Message\ExtranetMessage': async
        'Symfony\Component\Mailer\Messenger\SendEmailMessage': async

I need to send email with the symfony/mailer component to the email queue.

public function contact(Request $request, MailerInterface $mailer)
{
    if($request->isXmlHttpRequest())
    {
        //dd($request->request->all());
        $body =
            'Nouveau message depuis le front<br />
             Nom = '.$request->request->get('nom').'<br />
             Prénom = '.$request->request->get('prenom').'<br />
             Société = '.$request->request->get('societe').'<br />
             Email = '.$request->request->get('mail').'<br />';

        $email = (new Email())
            ->from('from@email.tld')
            ->replyTo($request->request->get('mail'))
            ->to('$request->request->get('mail')')
            ->subject('test')
            ->html($body);

        $mailer->send($email);

        return new JsonResponse('OK', 200);
    }
}

How can I add the binding_key to the mailer in order to let rabbitMQ know how to handle the email ?

Allright, I found the answer while searching for the complete messenger configuration reference.

In order to process messages without binding key, a default_publish_routing_key entry has to be added. The configuration now looks like :

    transports:
        # https://symfony.com/doc/current/messenger.html#transport-configuration
      async:
        dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
        options:
          exchange:
            name: myexchange
            type: direct
            default_publish_routing_key: email
          queues:
            email:
              binding_keys:
                - email
            extranet:
              binding_keys:
                  - extranet
        # failed: 'doctrine://default?queue_name=failed'
        # sync: 'sync://'

    routing:
        # Route your messages to the transports
        'App\Message\ExtranetMessage': async
        'Symfony\Component\Mailer\Messenger\SendEmailMessage': async

This allows the messenger component to process messages event if they don't have any queue specified.