Thank you for your reply - a short last question:
Is there a difference between MRPE and “Integrate Nagios plugins”" (Setup → Services → Other Services → Integrate Nagios plugins"?
MRPE = running on the monitored client machine
Integrate Nagios plugins = Running on the Monitoring server and bind the checks to the client machines?
Like “check if IP/DNS xy is on a spamlist”, running on the monitoring server and bind this check to the client server with that DNS/IP?