Email alerts from the Enterprise dashboard will notify you of an instance becoming available or unavailable. These alerts will fire when an instance comes on or offline within the dashboard.
Alerts will fire most commonly as an instance was restarted, however, this is not always the case.
The enterprise dashboard will mark an instance as offline when it has been able to retrieve data for that instance after 5 attempts, this can be due to:
An instance restarting
Network issues on the machine
The instance being limited by CPU or Memory
You can tune the data ping rate and the number of failed attempts within the Enterprise Dashboard settings.