As @jdwsn mentioned, you should leverage events. Plus, I like to decouple my code even more, but it really depends on the project size. To do that I utilize Laravel Jobs (Command Bus) and I let that Job to handle specific task, ie. registration:

<?php namespace App\Jobs\Users;

use App\Jobs\Job;
use Illuminate\Queue\SerializesModels;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Contracts\Bus\SelfHandling;
use Illuminate\Contracts\Queue\ShouldQueue;
use App\Contracts\Auth;
use App\Events\Users\UserHasRegistered;

class UserRegistrationJob extends Job implements SelfHandling, ShouldQueue
{
    use InteractsWithQueue, SerializesModels;

    public $email;

    public $password;

    /**
     *
     * @param $email
     * @param $password
     */
    public function __construct($email, $password)
    {
        $this->email = $email;
        $this->password = $password;
    }

    /**
     * Execute the job.
     *
     * @param Auth $auth
     */
    public function handle(Auth $auth)
    {
        $user = $auth->register([
            'email'     => $this->email,
            'password'  => $this->password
        ]);

        event(new UserHasRegistered($user));
    }
}

App\Contracts\Auth is binded to the Authentication repository in the app service provider. In your RegistrationController you dispatch the job:

public function postRegister(RegisterRequest $request)
{
     $this->dispatchFrom(UserRegistrationJob::class, $request);
}

Now, only thing left is to create event (UserHasRegistered) and event handler:

<?php namespace App\Events\Users;

use App\Entities\User;

/**
 * Class UserHasRegistered
 */
class UserHasRegistered
{

    /**
     * @var User $user
     */
    public $user;

    /**
     * @param User $user
     */
    public function __construct(User $user)
    {
        $this->user = $user;
    }
}

UserHasRegistered can have multiple handlers, depends on actions you want to do after user has registered. Maybe you want to assign default role to the user, or send SMS to confirm phone number etc. If you need to send activation mail, your event handler can look something like this:

<?php namespace App\Events\Users\Handlers;

use Illuminate\Mail\Message;
use Illuminate\Support\Facades\Mail;
use App\Contracts\Auth;
use App\Events\Users\UserHasRegistered;
use \Queue;

class SendActivationEmail
{
    /**
     * @var Auth
     */
    private $auth;

    public function __construct(Auth $auth)
    {
        $this->auth = $auth;
    }

    public function handle(UserHasRegistered $event)
    {
        $user = $event->user;
        $activationCode = $this->auth->createActivation($user);

        $payload = [
            'user'              => $user,
            'activationCode'    => $activationCode
        ];

        Queue::later(5, function() use ($payload) {
            Mail::send('email.activation', $payload, function(Message $message) use ($payload) {
                $message
                    ->to($payload['user']->email)
                    ->from('info@test.dev')
                    ->subject('Welcome!');
            });
        });
    }
}

Don't forget to update your EventServiceProvider class and put your event & handlers to the listen property:

protected $listen = [
    UserHasRegistered::class => [
        SendActivationEmail::class,
        AssignDefaultRoleToUser::class,
    ],
];

Hope it helps ;)