[ale] Is there an open source product similar to Exchange?

[FishR at bellsouth.net (Ryan Fish)]

MessageOne wonders, why ask here? 

Also, Exchange is MS' mail server app; is that what you meant to say or
did you mean Outlook?


I am in search of an app that runs on a Linux backend but allows Windows users to access it.  Basically, it needs to offer features similar to Exchange on the server side and be accessible by something like Outlook (or whatever mail client) on the user end.
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