I'm just starting to dabble in this stuff too - and I was looking for something simple that would work on a server, using my POP3 accounts. I'm not sure I'm ready to put toegther my own email server just yet...
I found this program called LANSuite. like some of the others here it is free for personal use or up to five users.
I found it pretty easy to set up and collect - the only thing I still need to do is set up the integrated webmail facility. In a nutshell it collects you POP3 internet mail, and from there you can send things off to different internal mailboxes. So for example, I have domain name forwarding for my main email account, so if it sees my name in the to line then it sticks it in my mailbox.
You tell it how often you want it to collect your mail, then you use your regular mail program to collect it on your internal network from LanSuite.
The cool bit (which I haven't made work yet, as I need to figure out port forwarding on my router properly) is the webmail feature. LANSuite also has an inbuilt webserver, and part of that allows you to login to your own server from any web browser, and see your mailboxes from there. They've just brought out a new version but I haven't tested it yet! Available at
http://software602.com/