Send E-Mail over local network

Hey Everyone

Can someone direct me to a detailed tutorial or good service of accomplishing this? Our internet drops from time to time and the staff e-mail each other all the time. So if I can get the email to run over the local network for the staff that would be amazing. Everyone here is on a workgroup.

I have clearOS 6.2 community as a firewall/server

Not sure about any other details I need to add.

Thanks