Laravel use MailMessage in Mailable

Publish notifications from vendor and you can send it in markdown

    public function __construct($user)
    {
        $this->user = $user;

        $this->message = (new MailMessage)
            ->greeting('Bonjour '.$user->name)
            ->line('Nous vous remercions de votre inscription.')
            ->line('Pour rappel voici vos informations :')
            ->line('Mail: '.$user->email)
            ->line('Password: '.$user->password);
    }

    /**
    * Build the message.
    *
    * @return $this    
    */
    public function build()
    {
         return  $this->markdown('vendor.notifications.email', $this->message->data());
    }

I used this in Laravel 8, not sure if it is still compatible with previous versions of Laravel. The point is to use the html method in the Mailable.

<?php

namespace App\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Queue\SerializesModels;

class MyMail extends Mailable
{
    use Queueable, SerializesModels;

    public function build()
    {
        return $this->subject('My Subject')
            ->html((new MailMessage)
                ->line('The introduction to the notification.')
                ->action('Notification Action', url('/'))
                ->line('Thank you for using our application!')
                ->render()
            );
    }
}

You are mixing 2 separate Laravel concepts, Notifications and Mailers. Notifications can be Mailers, but Mailers can not be Notifications.

The MailMessage class is a notification message, but cannot be the message for a Mailable. To send a MailMessage mailer you should extend the Notification class:

<?php

namespace App\Notifications;
use Illuminate\Notifications\Notification;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Queue\SerializesModels;
use Illuminate\Bus\Queueable;

class WelcomeNotification extends Notification implements ShouldQueue
{
    use Queueable, SerializesModels;

    public $user;

    public function __construct($user)
    {
        // The $notifiable is already a User instance so not really necessary to pass it here
        $this->user = $user;
    }

    public function via($notifiable)
    {
        return ['mail'];
    }

    public function toMail($notifiable)
    {

        return (new MailMessage)
            ->greeting('Bonjour '.$this->user->name)
            ->line('Nous vous remercions de votre inscription.')
            ->line('Pour rappel voici vos informations :')
            ->line('Mail: '.$this->user->email)
            ->line('Password: '.$this->user->password);
    }

}

Also, see Laravel's ResetPassword notification as example.

To send the notification to a user:

$user->notify(new WelcomeNotification($user));

In this manner you can create generic mail messages using the default mail notification template.