If you're looking for a great e-mail client for Linux, you don't have to look very far. Kmail comes installed by default if you install KDE. It's a great application that is similar to (but better than) Microsoft's Outlook.

If you're looking for something a little simpler that only has your basic email/address book/rss functionality, I highly recommend Mozilla Thunderbird. Thunderbird is also available for OS X and Windows, and is a recommended alternative to Outlook's e-mail functionality as well.

There are many other email apps out there, but either of these two will likely meet your needs.