At BradsDeals.com, we use Google Apps Email. It's a private-label version of Gmail that is customized to use an @bradsdeals.com address. Aside from being free (my favorite four-letter word), here's why:

-BradsDeals email is now on Google's infrastructure so it can't get more safe, secure or reliable
-The interface includes Google Chat and Calender so our team can quickly instant message and plan; this is crucial because each great deal involves a ton of research and coordination
-Google hosted email syncs easily with our iPhones and other mobile devices
-Setup was a breeze and there's no learning curve since most people are familiar with Gmail

There are also some powerful advanced tools like email forwarding, list management and the ability to have Google Apps Email suck in email from any other provider (AOL, Yahoo, Hotmail, etc.).

Best of all, you save money all-around: no Exchange Server, no Outlook on each computer and no additional hosting costs