email notification for five star alerts

I'm not exactly sure what you're trying to do. Do you need to setup a specific notification for sensors rated with five stars? In that case you can create a Library in PRTG, containing a node that picks all sensors filtered by 5 stars priority for example and configure there a notification that applies to all those sensors.

Kind regards.

Thank you sincerely for your reply. I have created a node as advised with all the sensors rated with 5 stars filtered. Now, Please apprise me on how to create email notification for the node if the sensors goes to down state

That's great. There's a separate tab "Notifications" in your library. What you need to do there is to add a State Trigger ("When sensor is down for x seconds, perform notification xyz"). This will apply to all sensors inside the library, current ones and also future sensors that match the library's filter criteria (all 5 stars sensors in your case).

Kind regards.