When you send a message to user(s), it is sent from your PC to port 6051 of the REACT host server, using a TCP call. The security server determines the name of the user’s PC (from when they logged in) and sends the message to the user's PC on port 6061 also using a TCP call.
If some of the users are not receiving the message verify that port 6061 is open from the server to the users' PC as TCP .