EMQ is described as 'X is an open source MQTT 5.0 broker for IoT in 5G era, massively scalable and highly available clustering, running anywhere from edge to cloud'. There are four alternatives to EMQ for a variety of platforms, including Linux, Mac, Windows, Amazon Web Services and BSD. The best alternative is mosquitto, which is both free and Open Source. Other great apps like EMQ are VerneMQ (Free, Open Source), CrystalMQ (Paid) and HiveMQ (Free Personal).
Eclipse Mosquitto is an open source (EPL/EDL licensed) message broker that implements the MQTT protocol versions 5.0, 3.1.1 and 3.1. Mosquitto is lightweight and is suitable for use on all devices from low power single board computers to full servers.
VerneMQ is an open source, high-performance, clusterable MQTT broker. It scales horizontally and vertically on commodity hardware to support a high number of concurrent publishers and consumers while maintaining low latency and fault tolerance.