Not sure if this is helping,
If you use a distribution like Redhat Fedora Core 1 or 2, sendmail and pop etc it is all on the distribution and ready to be ticked off (and installed). Even squirell web mail will work then (which is webmail ala hotmail)
Use webmin (google: webmin) to setup the mail server. it makes it easy.
If the users are on a local area network, you simply limit the IP addresses that can use sendmail to stop spammers using the server. If these users are dial up people using other ISPs it is harder since they might receive random IP addresses. If you want an open relay (no configuration whatsoever) it is easy to set up using sendmail and webmin, but spammers would love you, and the rest of the world hate you. But you can try to install AUTH using http://www.simpaticus.com/linux/howt...UTH-HOWTO.html
. These will make users provide passwords when using pop etc also when sending mail. But some setup in each users Outlook express is required then (they have to tick off some options).
Hope it helped a bit..