I think there are two possibilities:
1. Either email sending is completely broken in your Moodle. You should test this by trying some other Moodle things that normally send emails (Forum subscriptions, Messaging system).
2. Or, this is a problem with the quiz settings that have changed. Check your setup against https://docs.moodle.org/38/en/Quiz_submission_email_notification.
https://docs.moodle.org/38/en/Quiz_submission_email_notification
1. Either email sending is completely broken in your Moodle. You should test this by trying some other Moodle things that normally send emails (Forum subscriptions, Messaging system).
2. Or, this is a problem with the quiz settings that have changed. Check your setup against https://docs.moodle.org/38/en/Quiz_submission_email_notification.
https://docs.moodle.org/38/en/Quiz_submission_email_notification