Fatal error: Uncaught Error: Class "Drupal\Core\Mail\MailManager" not found in /home/www/drupal8/test/modules/contrib/mailsystem/src/MailsystemManager.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/mailsystem/src/MailsystemManager.php on line 20