Error 500 when including controller file in Route
If that is the entirety of your controller, then you are missing use statement for ControllerBase.

namespace Drupal\mailing_list\Controller;

use Drupal\Core\Controller\ControllerBase;

class MailingListController extends ControllerBase {
  public function mailingListList() {
    $output = array('table', array('header' =&gt; '1'));
  }
}


You also need to return a Response or renderable array from the controller method the route calls.

Example:

namespace Drupal\mailing_list\Controller;

use Drupal\Core\Controller\ControllerBase;

class MailingListController extends ControllerBase {
  public function mailingListList() {
    $output = [];
    $output['#markup'] = $this-&gt;t('Hello!');
    return $output;
  }
}


See: https://www.drupal.org/docs/8/api/routing-system/introductory-drupal-8-routes-and-controllers-example

If that does not work you need to post the full error you are getting.