Dennis @ PerfectForms
In the Edit Forms tab click on the form, click on instances, pick an instance and click details. It will list outstanding notifications. This may or may not help you locate the notification that is trying to send to an invalid address.
Best practices is to set all notifications as part of the workflow on the join paths between stages. Among other things, it makes all of the notification behaviors easy to find. If you are using roles, check all of the roles in the form.