The Mosquitto MQTT is MQTT compliant:
MQTT Version 3.1.1: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html
The MQTT version 5.0 is the next development step in the evolution of MQTT. Learn about the Eclipse Foundation. Major bugs have been erased. For example in many cases errors led to disconnecting the client from the broker. With v.5.0 negative acknowledgements help to work out ways to solve the problem.