2 entries are registered on port 323. It is a well-known port (0–1023) — assignments here are IANA's, and a service found on one is usually the service that was assigned it.
The Internet Message Mapping Protocol (IMMP) is designed to facilitate message synchronization and mapping between messaging systems, aiding interoperability across disparate email services and platforms. Supporting both TCP and UDP transports, IMMP aligns with multi-environment messaging needs and provides the foundation for translating message metadata effectively. Its use case centers on connecting heterogeneous communication systems without standardized protocols for efficient message delivery.
The registry says what 323 is registered to. What is listening on it right now is decided by the host, and only the host can be asked. Each of these prints the process behind the socket.
linux
ss -ltnp | grep ':323\b'
macos
lsof -nP -iTCP:323 -sTCP:LISTEN
windows
netstat -ano | findstr :323
On Linux and Windows the process name comes back with the socket; on macOS lsof needs sudo to name a process that isn't yours. Match what it prints against the 2 entries above.
A port number is not a service. Anything may listen on 323 — what you actually met is decided by the host, not the registry. If a service is missing here, it belongs in the registry.