Well, if you are using windows, the ideal thing is Exchange. I don't mean to hurt the open-source zealot in you, but its the truth, Exchange offers lots of really powerful features. If you can't get a copy for a reasonable price, then I think you are gonna have to settle for pretty bad stuff. I tried setting up a windows email server a while ago too and had a lot of trouble finding quality software. If i rememebr correctly, Imail wasn't too bad (
www.ipswitch.com). Anyway, good luck.