Thunderbird

From the site:

Thunderbird is a free, open-source and cross-platform mail client for most operating systems including, but not limited to, Windows, Linux and Macintosh. It is based on the Mozilla codebase. It is a robust and easy to use client, similar to competing products like Outlook Express, but with some major advantages such as junk mail classification.

Download Mozilla Thunderbird (Mac or Linux)
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