Target [Illuminate\Contracts\Mail\Factory] is not instantiable.

I am getting the following error when I run my queue

local.ERROR: Illuminate\Contracts\Container\BindingResolutionException: Target [Illuminate\Contracts\Mail\Factory] is not instantiable. in ....../vendor/illuminate/container/Container.php:982

in my bootstrap/app.php I added

$app->configure('mail');
$app->alias('mailer', Illuminate\Mail\Mailer::class);
$app->alias('mailer', Illuminate\Contracts\Mail\Mailer::class);
$app->alias('mailer', Illuminate\Contracts\Mail\MailQueue::class);

$app->configure('queue');

$app->register(Illuminate\Mail\MailServiceProvider::class);
$app->register(Illuminate\Queue\QueueServiceProvider::class);

This is how I try to send my mail:

Mail::to($user)->locale(App::getLocale())->send(new Register($user));

and this is the Register class:

<?php

namespace App\Mail;

use App\Models\User;
use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;

class Register extends Mailable implements ShouldQueue
{
    use Queueable, SerializesModels;

    /**
     * The user instance.
     *
     * @var User
     */
    protected $user;

    /**
     * Create a new message instance.
     *
     * @param  User $user The user instance.
     * @return void
     */
    public function __construct(User $user)
    {
        $this->user = $user;
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->view('emails.users.register');
    }
}

Sending mails without queueing them works perfectly fine. Am I doing something wrong, or is this a bug?

laravel/lumen-framework is version 7.x - illuminate/mail and illuminate/queue are both 7.3.0

Cheers ;)