I would first consider Openexchange Server, which is an open source collaboration and integration server that offers much of the same functionality you already have under Windows with Microsoft Exchange.
You can receive professional support from both Novell and Netline, which hosts the Open-Exchange Web page.
Also, you may want to consider Versora to migrate the data from Exchange to OpenExchange Server. Versora offers a full suite of tools for migrating from Windows servers to Linux servers. They specialize in migration from Microsoft Exchange to Open Exchange.
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