Sample service provider for extending Laravel's mail provider.

ExtendedMailServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Mail\MailServiceProvider;

class ExtendedMailServiceProvider extends MailServiceProvider
{
    /**
     * Extended register the Swift Transport instance.
     *
     * @return void
     */
    protected function registerSwiftTransport()
    {
        parent::registerSwiftTransport();

        app('swift.transport')->extend('spark', function($app)
        {
          return new \App\Mail\SparkPostTransport('theclient', 'thekey123');
        });
    }
}