Nagios Support Forum

Is there a way to only receive email alerts when Warning or Critical levels are reached instead of when levels are ok?

There is not a way to setup Email Alerts in NNA to only send when the status changes. It is a feature that will be added to the next major release of NNA.
Check here for when the next major release may happen.
https://www.nagios.com/roadmaps/

If you have a Nagios XI server, you can send the Alerts to that server and then it would act the way you want.