Issue #2865177 by jhedstrom: Pass the digest notifier along to the mail system

   *   The entity type. Leave empty for global digests.

   * @param string|int $entity_id

   *   The entity ID. Leave empty for global digests.

   * @param \Drupal\message_digest\Plugin\Notifier\DigestInterface $notifier

   *   The digest notifier plugin ID used for this digest.

   * @param string $formatted_message

   *   The formatted message.

   */

  protected function deliverDigest(UserInterface $account, $entity_type, $entity_id, $formatted_message) {

  protected function deliverDigest(UserInterface $account, $entity_type, $entity_id, DigestInterface $notifier, $formatted_message) {

    $params = [

      'body' => $formatted_message,

      'entity_type' => $entity_type,

      'entity_id' => $entity_id,

      'notifier' => $notifier,

    ];

    // @todo Use the message_notify sender service.

    // @see https://www.drupal.org/node/2103013

    $this->moduleHandler->alter('message_digest_view_mode', $context, $notifier, $account);

    if ($context['deliver']) {

      $formatted_messages = $this->formatter->format($context['messages'], $context['view_modes'], $account);

      $this->deliverDigest($account, $context['entity_type'], $context['entity_id'], $formatted_messages);

      $this->deliverDigest($account, $context['entity_type'], $context['entity_id'], $notifier, $formatted_messages);

    }

  }