Email notifications are not working despite configuration being correct.
Setup a test environment and test the specific Email notification that is broken. We have many Notification options that are available within the software.
Send a test email using the "Send Test Email" option.
From this point you will be able to send notifications to any Email you desire. Test the notification the specific customer has had an issue with to determine cause or if issue can be reproduced. You will most likely need to recreate their exact environment and test to ensure you have the best chance to reproduce the issue.