Port 1883: MQTT
Port 1883 is MQTT, a lightweight messaging protocol used by smart-home and IoT devices.
What runs on port 1883
MQTT brokers listen on TCP 1883 and relay short messages between devices that publish and subscribe to topics, such as a sensor reporting temperature. Mosquitto is the most common broker. Plain 1883 is unencrypted; the TLS version runs on 8883.
On a home network
Home Assistant setups with the Mosquitto add-on, Zigbee2MQTT, Tasmota and Shelly devices, and some solar inverters and EV chargers use MQTT on 1883.
If port 1883 is reachable from the internet
A broker on the internet without a password lets anyone read every message and send commands, which can mean switching devices on or off. Researchers regularly find large numbers of open MQTT brokers leaking home and industrial data. Keep 1883 on the LAN.
How to check whether it is open on your network
Check that your broker requires a username and password, and that no router rule forwards 1883. A LAN port scan will show which device is the broker.
To find out whether port 1883 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.