Hi Tony
The system rely on your mail server for sending mails, so if your mail server didn't work at the time for some reasons, both you and your registrants won't receive email.
If you haven't received any emails before, there might be something wrong with mail settings. But if you received some of them and not some of the others, mostly the issues come from your mail server. You can contact your hosting provider and ask them to check the emails log to see whether the emails are sent or not
Tuan
