Some email notifications quit working

Some of our forms are still sending out email notifications and others have quit. How can I troubleshoot this?

We have checked the mail logs, and all notifications show as sent: https://www.jotform.com/help/293-How-to-View-All-Your-Form-Email-History

Please make sure to check your spam folder, if you find them there, mark them as NOT SPAM. If the issue persists, you may try using your SMTP settings: https://www.jotform.com/help/244-How-to-Setup-SMTP-for-a-Form