Email server

You need a server to install it eg Windows, Linux or Netware
e.g.
http://www.pmail.com/overviews/ovw_mercury.htm
http://www.emailman.com/unix/servers.html
http://www.microsoft.com/windowsserver2003/sbs/default.mspx

If you just want to distribute emails in-house and use a ISP for external mails you can search the internet for "nail servers" and download one. There are freeware/shareware servers avai;able such as Hamster, Jana Server, Mercury32 etc ...
You do not need a dedicated server to run these.
If your network is rather large 50+ I would however recomend a dedicated server (machine)