[wlug] LAN mail server

I am looking for suggestions and maybe even links, on info on setting up a LAN mail server. It needs to collect ISP email and send email to an ISP. So, collect @orcon.net.nz email, store it in imap or what ever and send it back to orcon. No IP address or domain name, just ISP email. I am a complete email server nub, so any help would be great. Mike