Issue #2971692 by opdavies, git commit -m Issue

private_message.module

@@ -10,6 +10,7 @@ use Drupal\Component\Utility\Xss;
 use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Mail\MailFormatHelper;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
@@ -469,7 +470,7 @@ function private_message_mail($key, &$message, $params) {
     $message['from'] = \Drupal::config('system.site')->get('mail');
 
     $message['subject'] = SafeMarkup::checkPlain($message['subject']);
-    $message['body'][] = Xss::filter($body);
+    $message['body'][] = MailFormatHelper::htmlToText(Xss::filter($body));
   }
 }