Recently, I stopped receiving email notifications when students received their certificates. My Moodle installation is running on Bluehost.
Bluehost was finally able to determine that there is a change in the way they verified emails in their firewall. In order to send an email, you will need to have a valid From: email defined in the email section of your cpanel. Prior to this change, I was using a "noreply@...." email that was not defined in the cpanel for my email accounts.
Interesting, thank you for pointing it out.