Two recommendations to get you started, and they are mutually exclusive:
Google Apps will handle your mail and give you a Gmail-like interface, which can be nice if you're inexperienced with running your own mail server. There are caveats to running an MTA which can land you in spam hell fairly quickly, and I try to recommend outsourcing mail when possible. You can have fifty users before you have to pay, if I recall.
The other option, which I've never used, is
Citadel. At first glance it appears to do all that you are after, but I can't vouch for it.