The client supports version 3.1 and 3.1.1 of the [MQTT](https://en.wikipedia.org/wiki/MQTT) protocol. Make sure that the correct version is set with `make menuconfig` -> "Component config" -> "ESP-MQTT Configurations" -> "Enable MQTT protocol 3.1.1".
The client adheres to version 3.1.1 of the [MQTT](https://en.wikipedia.org/wiki/MQTT) protocol. Make sure that your broker supports and is correctly configured for version 3.1.1. The client is backwards incompatible with brokers running MQTT 3.1.
!!! note "Unsupported transport modes"
Even though the MQTT configuration offers the transport modes Websocket and Websocket Secure, they are currently not supported by the `mqtt` module.