You can setup your own email server internally, or use an IM client running on the computers. Messaging systems can hold messages like email, but you'd need to have a server setup to run them. If you just send an instant message to another PC without a server to hold and manage them, it will just go no-where if the other PC is off or if the IM program is not running.
http://www.softpedia.com/get/Internet/Chat/LAN-Messaging-Clients/
http://www.vitalsoundlab.com/