Alternatives to M$ Exchange?

How about Postpath?

Related articles:

Can open source messaging servers replace Exchange?

Article about Postpath

Another article "Exchange, but without its problems (like Microsoft :-)? Try PostPath"

And another - PostPath intros plug-compatible Exchange Server
Linux-based PostPath looks like Exchange

Messaging vendors tout Exchange alternatives, Gordano, PostPath design products that look just like the Microsoft software.

It is impressive that it claims to integrate with AD, speaks native Exchange protocol, works with Exchange-dependent apps such as BES, lower costs, better scalability, cool AJAX web client, and leverages all the cool Linux and Open Source benefits, but ...

How about future M$ VoIP and unified messaging integration? Also presence functionalities? Not to forget Cisco CallManager integration?