Hm. Zimbra does _that_ very well IMO. Supports IE+FF+Safari, at least for the webmail-stuff.
I'm not sure if the Open-Source version actually supports the Outlook-stuff (we use the commercial version and I don't use Outlook anyway...).
I'd give Zimbra a try. It's relatively easy to setup, at least for a demo-case where you are not interested in customizing all the logos.
Rainer
One thing I have found interesting with Zimbra is that you can use their open source community edition and then get the outlook connector as an add-on for the users you want/need. We have a full licensed version due to the request to keep using outlook from most people. I would say their web based client is very good, but others are good too. We looked at Scalix and found it a little more complicated and complex than we were comfortable with going with. Having components that I was familiar with from prior experience in the linux world was a big help vs learning new.
Andrew