drupal/core/modules/contact/contact.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the contact module.
 */

use Drupal\Core\Language\Language;

/**
 * Implements hook_install().
 */
function contact_install() {
  $site_mail = \Drupal::config('system.site')->get('mail');
  if (empty($site_mail)) {
    $site_mail = ini_get('sendmail_from');
  }
  $config = \Drupal::config('contact.form.feedback');
  // Update the recipients if the default configuration entity has been created.
  // We should never rely on default config entities as during enabling a module
  // during config sync they will not exist.
  if (!$config->isNew()) {
    \Drupal::config('contact.form.feedback')->set('recipients', array($site_mail))->save();
  }
}