Microsoft Exchange equivalent?

[Marius Andreiana <mandreiana rdsnet ro>]

Hi!

I've searched for a free ( as in speech ) equivalent of Microsoft
Exchange on freshmeat, sourceforge and google, but without success. I've
found only proprietary products so far: IBM's Lotus Domino,
Hewlett-Packard's OpenMail, Bynari's Insight Server and Suse's Open
Exchange.

Is there any free software that comes close to this?
Thanks
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