You can create an alter that will notify the end user when anything is changed in the system which will initiate an email notification when an email message is added to the existing incident. You can also create an alert trigger to on a specific field that changes in the incident when an email is added as well. We personally have an email notification setup for the [Assigned To] user that is notified when an item is changed in which it initiates an email notification when new email messages are added to the items.

Thank you,