Mozilla Thunderbird (Linux)

From Mozilla:

If you like Firefox's tabbed browsing, you're going to love tabbed email. Thunderbird 3's tabbed email lets you load emails in separate tabs so you can quickly jump between them. Search results open in a new tab too. New tools like our timeline and filtering tools will help you pinpoint the email you're looking for, whether it's the one from yesterday, last month, or several years ago. Customize Thunderbird however you want, whether it's to make you more productive or to make Thunderbird more fun. You can choose from hundreds of add-ons like Personas to change Thunderbird's look. You can now find and install new add-ons directly from the Add-ons Manager. All you need to provide is your name, email address, and password and Thunderbird will find your email settings and set up your email accounts for you.