Sending mail from drupal 8

drupal_8_send_email.php

<?php

// To be passed to hook_mail
$params = array();

// To be passed to hook_mail
$key = 'my_mail_key';

$langcode =  \Drupal::languageManager()->getDefaultLanguage();
\Drupal::service('plugin.manager.mail')
  ->mail('module_name', $key, $to, $langcode, $params);

module_name.module

<?php

/**
 * Implements hook_mail().
 */
function module_name_mail($key, &$message, $params) {
  switch ($key) {
    case 'my_mail_key':
      $message['subject'] = 'New mail for you';
      $message['body'][] = 'Message body';

      break;
  }
}