mailjet-8.x-2.7/modules/mailjet_campaign/src/Controller/CampaignCallbackController.php

<?php

/**
 * @file
 * Contains \Drupal\mailjet_campaign\Controller\CampaignCallbackController.
 */

namespace Drupal\mailjet_campaign\Controller;

use Drupal\Core\Controller\ControllerBase;

class CampaignCallbackController extends ControllerBase
{

    public function callback()
    {
        _mailjet_campaign_alter_callback();
        die();
    }

}