My home network is like this:

5-6 devices (wirelessly) -----> Router ---> ISP Modem ---> Internet
File server-Ubuntu 12.04 (wired)--^

All devices are part of the same workgroup. I'm interested in learning more about networking and I would like to set up an internal mail server that allows all devices to send emails to each other.

I've found some good tutorials on the web but most are too advanced for my needs. I don't need to receive email from outside, just internally... for starters. I found this one tutorial which seems fit for my needs but its 5 years old. Also I'm a little confused about domains. In order to use email internally, do I need to register a domain? And it seems I'd have to set up a DNS server (probably on the file server)...won't that screw up the internet access to the network if I set up my own DNS server?

I'm just experimenting since these things interest me, so if you can be patient and explain...please help.