Welcome To Drupal 8
Drupal is a CMS (Content Management System) open-source written in PHP. Drupl 8 is the latest version of Drupal successor of Drupal 7 popular for its security, flexibility and scalability, used on the complex high traffic web applications as well as small showcase sites. Drupal 8 is comes with over 200 new features and improvements including native full responsive support, multilingual support, REST Full web service. Drupal 8 was created in PHP using Symfony 2 Framework and TWIG templating system.
This web site is a set of tutorials help to new Drupal 8 developers and persons who want to be a Drupal 8 developer or a webmaster. The level of the tutorials are start from very basic (beginners) and can be very complex (Experts). I hope the site is useful for you and you are welcome to share your knowledge, ideas and experience. Thank you.
You have several mthodes to override 403 / 404 System pages.
Method 1. Set 403 / 404 pages.
You can set the 403 /404 page on "Basic site settings" page
Admin -> Configuration -> System -> Basic site settings
An email is classified as spam or not spam on the receiving end, not the sending end. But you can improve
$result = array(
'#markup' => 'Hello. This is my First Page',
$renderer = \Drupal::service('renderer');
$html = $renderer->render($result);