Zimbra is possibly the closest thing we have found to being the Holy Grail of groupware for linux.
It is a "Web 2.0" application that uses AJAX to provide a very smooth user experience. It's not just another webmail package, but a full groupware suite comparable to MicrosoftExchange. It has many seriously nice features, like date and time "sensing" in an email, to allow you to click on the phrase "next Thursday" in an email and have a Calendar appointment created for this date.
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