Integration testing the mail dispatching in Laravel 5.1

When using the Mail facade in Laravel it is not so easy to test the output of the parsed mail template. Like I received the error Method Mockery_0__vendor_Swift_Mailer::getTransport() does not exist on this mock object. I ended up in listening to the mailer.sending event:

    public function testRegistrationMailIsSend_afterSubmittingForm()
        // flag for closure has been called
        $mailerAssertionHasBeenCalled = false;

        // receive every Event::fire method and pass the reference from the outer scope into the closure
        Event::shouldReceive('fire')->andReturnUsing(function($event, $params) use (&$mailerAssertionHasBeenCalled) {
            // filter only the mailer.sending event
            if ($event != 'mailer.sending') {

            // reference will be modified
            $mailerAssertionHasBeenCalled = true;
            // Swift_Message; Illuminate\Mail\Mailer::sendSwiftMessage
            $msg = $params[0];

            $this->assertEquals('Verify your e-mail account', $msg->getSubject());
            $recipients = $msg->getTo();

            $this->assertTrue(array_key_exists('', $recipients));
            $verificationKey = Registration::first()->verification_key;

            // assert registration key is present in parsed template
            $this->assertContains('/registration/verify-email?key=' . $verificationKey, $msg->getBody());

        // visit our registration controller
            ->submitForm('Register', ['email' => ''])
            ->see('Mail has been sent');

        // make sure that our closure has been called