If you're coming from Windows to Linux, you might as well try something completely different and new. Or you combine the reliability and creativity of your new operating system with the easy to use interface you know from Windows, like these email programs do.
This wonderful email client, calender and groupware application not only looks like Outlook, it also matches Microsoft's email program in features and functionality.
Mozilla Thunderbird is a fully featured, secure and very functional email client and RSS feed reader. It lets you handle mail efficiently and with style, and Mozilla Thunderbird filters away junk mail too.
Integrated with the nice KDE desktop, KMail is powerful, but easy to learn use, especially if your're coming from Windows.
Balsa is part of the Gnome desktop environment (which is just as nice as KDE), but it does not yet equal KMail in advanced features.
Sylpheed is a friendly email client with a particularly easy to use interface. There are a few things that Sylpheed does better than Balsa, and a few more where Balsa has an advantage.
Alpine is powerful console email program that makes you use email productively with automation aplenty and nary a distraction.