contact_entity.module

<?php

/**
 * @file
 * Contains contact_entity.module.
 */

use Drupal\Component\Utility\Html;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Entity\EntityInterface;

/**
 * Implements hook_help().
 */
function contact_entity_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the contact_entity module.
    case 'help.page.contact_entity':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Custom entity for contact') . '</p>';
      return $output;

    default:
  }
}

/**
 * Recommend for use, but not require...
 * Implements hook_mail().
 */
function contact_entity_mail($key, &$message, $params) {
   $options = [
    'langcode' => $message['langcode'],
  ];
  switch ($key) {
    case 'contact_message':
      // Need to send a formatted text.
      $message['headers']['Content-Type'] = 'text/html; charset=UTF-8; format=flowed; delsp=yes';
      $message['from'] = \Drupal::config('system.site')->get('mail');
      $message['subject'] = t('User have subscribed, his telephone: @telephone', ['@telephone' => $params['telephone']], $options);
      $message['body'][] = Html::escape($params['message'][0]['value']);
      break;
  }
}

/**
 * Create email only for created entity.
 * Implements hook_entity_insert().
 */
function contact_entity_entity_insert(EntityInterface $entity) {
  if ($entity->getEntityTypeId() !== 'contact_entity') {
   return;
  }
  $mailManager = \Drupal::service('plugin.manager.mail');
  $module = 'contact_entity';
  $key = 'contact_message';
  $to = $entity->get('email')->getString();
  $params['message'] = $entity->get('message')->getValue();
  $params['telephone'] = $entity->get('telephone')->getString();
  $language_code = $entity->language()->getId();
  $send_now = TRUE;
  $mail = $mailManager->mail($module, $key, $to, $language_code, $params, NULL, $send_now);
  if ($mail['result'] == TRUE) {
    drupal_set_message(t('Your message has been sent.'));
  }
  else {
    drupal_set_message(t('There was a problem sending your message and it was not sent.'), 'error');
  }
}

/**
 * Implements hook_mail_alter().
 */
function contact_entity_mail_alter(&$message) {
  // Id is module name plus key.
  if (!empty($message['id']) && $message['id'] == 'contact_entity_contact_message') {
    $reverse = $message['from'];
    $message['from'] = $message['to'];
    $message['to'] = $reverse;
  }
  // dpm($message); // For debug via devel.
}