Hm, I installed OpenHAB now on my RasPi, also Mosquitto. I think I begin to understand this. So MQTT is just some kind of "forum", and when in the thread "livingroom/temperature" something changes, OpenHAB recognizes it. The sensor sends just its data to the gateway and the gateway writes the MQTT thread?
Will the other way round work too, so OpenHAB writes data into a specific thread, the gateway recognizes it and sends a command so, say a servo node to move something?
If that all is correct, I sould be able to connect the 433 MHz things just like a normal sensor node. The receiver acts as a normal sensor and the sender could use the servo sketch to send some commands via 433 MHz band?