@Bruce Smith Welcome to Q&A forum!
IoT Hub is not a full-featured MQTT broker and does not support all the behaviors specified in the MQTT v3.1.1 standard.
Azure IoT Hub has built-in the device-facing endpoint for handling directly the MQTT protocol between the device and IoT Hub in the bidirectional manner.
All device communication with IoT Hub must be secured using TLS/SSL. Therefore, IoT Hub doesn't support non-secure connections over port 1883.
Please refer the Communicate with your IoT hub using the MQTT protocol for more details to understand how MQTT works with IoT Hub.
Do let us know if you have further queries.