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Sea Monkey was born out of the Mozilla Application Suite, using community-driven software to continue its development.
It's not as fully featured or easy to set up as Opera Mail or emClient, but once it's up and running, it can happily handle your daily email chores. It'll also sync Microsoft Exchange accounts or other web-based email accounts such as Gmail.
While functional, it's not the best looking of the clients, and it did take the longest to import messages. It also failed to respect my Gmail folder structure when dealing with other email accounts rerouted to Gmail.
Nevertheless, it's free forever and is available for Mac, Windows and Linux, so could be worth a look.
Screenshot: Ben Woods/ZDNet