materio-d9/web/modules/custom/materio_expo/materio_expo.module

<?php

/**
 * @file
 * Contains materio_expo.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function materio_expo_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the materio_expo module.
    case 'help.page.materio_expo':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('My Awesome Module') . '</p>';
      return $output;

    default:
  }
}

/**
* Implements hook_mail().
*/
function materio_expo_mail($key, &$message, $params) {
  switch ($key) {
    case 'expo_getmail_submitted':
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('[EXPO] mail submitted: @mail', array('@mail' => $params['email']));
      $message['body'][] = $params['message'];
      break;
  }
 }