UsersMailPreview.php

<?php 
namespace App\Mailer\Preview;

use Cake\Core\Configure;
use Cake\ORM\Locator\LocatorAwareTrait;
use DebugKit\Mailer\MailPreview;

class UsersMailPreview extends MailPreview
{
    use LocatorAwareTrait;

    public function validation()
    {
         $users = $this->getTableLocator()->get('Users');

         $user = $users->find()->first();
          return $this->getMailer("Users")
                      ->validation($user);
     }
}