Email on Lan

[wadeal]

What can I do to set up email on my LAN (Not connected to the net). I just want to be able to send messages between the computers in the LAN. I already have on my intended server Win XP's inbuilt IIS server, Apache Web server 2, Mysql and PHP extensions. Also, this is not much of an emergency but my access to the net is somewhat limited.
Thankyou, Wadeal