Does anyone know of an email client that works over a LAN but does not extend outside of the network?

I'm trying to figure out a way to give people the ability to email each other, but not other people outside the network. Is there such a thing as email that only connects to other people on the network?

Does anyone know of an email client that works over a LAN but does not extend outside of the network?
You can setup an internal email server and have them connect to that. That way they can send email around internally no problem. All you have to do is disable out going emails. This is done in different ways depending on what email server you decide to use. Also you could alternatively block the outgoing communications at a firewall. So really there are several ways you can do this. There are also several products out there that you can use for local instant messaging. If you need more information on this feel free to let me know.