At the cost of being strung up, let me recommend Exchange 2000. It is the best groupware server I have run. You get your shared calendar and contacts, and a nice Webmail server.
I have used a few Linux based solutions for pop/imap/web based mail, and they work great. E-Smith is hands down the easiest of the lot to get working. Once you add groupware functionality though, Microsoft seems to work the best (yes I have tried Notes, and don't like it).
For the downside on Exchange, it costs a lot. Expect to spend $700-$800 for a 5 user license, plus a Win2k Server license (another $700 or so) and you need a nice robust box. All in all, you figure on spending $2000-$2500 for around 5 users. As you scale up in users and sites, your costs will increase, but with an enterprise level budget it doesn't seem like that big of a hit.
Also, with Exchange Webmail you get IIS, so be prepared to patch like there is no tomorrow. Maybe you could get Webmail working with Apache, but I haven't seen it done.