Lumen with Mail - Unable to resolve NULL driver for [Illuminate\Mail\TransportManager]

I'm trying to send emails from Lumen 5.7, so as mentioned by many sources, I ..

1. added illuminate/mail and guzzlehttp/guzzle to my application
2. created "config/mail.php" and "config/services.php" from Laravel Repository
3. uncommented $app->withFacades();, registered Illuminate\Mail\MailServiceProvider::class and added $app->configure('services'); and $app->configure('mail'); before return $app; in bootstrap/app.php

4. added mailgun settings to .env

   MAIL_DRIVER=mailgun

   MAILGUN_DOMAIN=ssss

   MAILGUN_SECRET=xxxxx

5. tried to send an email Mail::raw('Raw string email', function($msg) { $msg->to(['x@x.com']); $msg->from(['x@x.com']); });

but still get this error message Unable to resolve NULL driver for [Illuminate\Mail\TransportManager].

P.S. This is my first experience with Lumen and I did search to resolve this problem for many hours, but I cannot.

Thanks in advance.