A message can be sent to all users who are connected to a computer using the Send Message command on the Computer menu in Server Manager. For example, you can do this before you disconnect one or more users or before you stop the Server service on that computer.
For a message to be sent and received, the Messenger service must be running on the computer sending the message and on the computers receiving the message.
For more information about sending messages, see "Sending a Message to Connected Users" in Server Manager Help.