I'm not a c++ person, but I do know that Domoticz expects JSON encoded data to be published to the MQTT channel, and the file you provided is just publishing a bit of plain text
Thank you very much, asjmcguire! This is indeed helpful.

I created a "Dummy hardware" with a virtual temp+hum sensor ...

Well to start with, Domoticz only responds to messages received on the topic domoticz/in

Yes, thank you. After fiddeling around I manged to make it publish the data to the "domoticz/in" topic.

When I subcribe to the MQTT domoticz/in topic with SSH in Raspberry Pi, I see:
Temperature received ...

Hi guys,
I am brand new to this stuff, so bear with me. I recently pruchased an Arduino Uno, along with a DHT22 temp+hum sensor and some RF24 radios.
They send the data to a Raspberry Pi (based on a Raspian Image file) with Domoticz. I have been following the instructions from http ...