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encyclo - 1:47 pm on Jan 20, 2010 (gmt 0)
The least bad, in my opinion, is AtMail - there's a commercial version (which is good but expensive), or a free community version (which is passably good, and free!): [atmail.com...]
There are a few possibilities, such as Squrrelmail, Uebimiau and others. Most of them are terrible. :)
[atmail.org...] (for the free version)