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Unlike some of the other software on this list, emClient offers a paid-for version for commercial use, in addition to the version that's free for home users. The commercial emClient costs around £32 ($49.95) for a lifetime licence, with 12 months of free upgrades.
emClient is closer to Opera Mail and Thunderbird than Sea Monkey. As well as providing support for syncing multiple accounts, it offers calendar, tasks and widget tools, and has a way to link into instant messaging.
Screenshot: Ben Woods/ZDNet