<?php
/**
* Implements hook_mail().
*/
function <module_name>_mail($key, &$message, $params) {
 $options = array(
   'langcode' => $message['langcode'],
 );
 switch ($key) {
   case 'create_article':
     $message['from'] = Drupal::config('system.site')->get('mail');
     $message['subject'] = t('Article created: @title', array('@title' => $params['node_title']), $options);
     $message['body'][] = $params['message'];
     break;
 }
}
/**
* Implements hook_entity_insert().
*/
function <module_name>_entity_insert(DrupalCoreEntityEntityInterface $entity) {
 if ($entity->getEntityTypeId() !== 'node' || ($entity->getEntityTypeId() === 'node' && $entity->bundle() !== 'article')) {
   return;
 }
 $mailManager = Drupal::service('plugin.manager.mail');
 $module = ‘<module_name>’;
 $key = 'create_article';
 $to = Drupal::currentUser()->getEmail();
 $params['message'] = $entity->get('body')->value;
 $params['node_title'] = $entity->label();
 $langcode = Drupal::currentUser()->getPreferredLangcode();
 $send = true;
 $result = $mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send);
 if ($result['result'] !== true) {
   drupal_set_message(t('There was a problem sending your message and it was not sent.'), 'error');
 }
 else {
   drupal_set_message(t('Your message has been sent.'));
 }
}