certificate-4.0.0-alpha1/modules/certificate_test/certificate_test.module

<?php

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Session\AccountInterface;

/**
 *
 * @param EntityInterface $entity
 * @param type $operation
 * @param AccountInterface $account
 * @return AccessResult $access
 */
function certificate_test_entity_access(EntityInterface $entity, $operation, AccountInterface $account) {
  if ($entity->getEntityTypeId() == 'certificate_test_entity' && $operation == 'certificate') {
    if ($account->getEmail() == 'certified@example.com') {
      return AccessResult::allowed();
    }
    else {
      return AccessResult::forbidden();
    }
  }
}