I don't recommend doing this. Instead, use your ISP's e-mail system. Configuring and managing e-mail servers is a pain, and if you do something wrong, you'll lose e-mail messages. That'll make everyone else really mad at you, and you'll have to scramble to figure out what happened.
If you really do need an internal mail server, Windows Server 2003 includes a very simple system built-in (Windows 2000 doesn't have what you need). Since I bet you're a small company, I'd check out Microsoft Small Business Server 2003. It includes Microsoft Exchange Server which is a very full-featured messaging and calendaring server, and it's really easy to set up.
Otherwise, you can look for third-party software. I haven't inventoried the different options available, but there are a lot. A good place to start is to look for free software at www.sourceforge.net. Try searching for "pop smtp server."
This was first published in May 2004