i have built a notification rule with the following setting. The messages when a service jumps to WARN or CRIT will also go out as desired.
Unfortunately no messages will be sent when it jumps back to OK. but this is important because this is an interface to our ticket system and an OK message should automatically close tickets.
There is also the ruleset “Notified events for services” in the section Monitoring Configuration where you can configure which events generate a notification from the core.