have you looked at openchange http://www.openchange.org/index.html
It's been a few years since I looked at it but the goal is to create a
exchange server replacement.

On Sat, Feb 8, 2014 at 11:34 AM, Nico Kadel-Garcia <[log in to unmask]> wrote:
> Whoops, sorry! I thought you were looking for an AD replacement, not
> an Exchange replacement.
>
> To replace Exchange, run, do not walk, to Google Apps for Business. It
> works very well, you don't have to maintain your own expert IT
> infrastructure to deal with the vagaries and backups and security of
> email handling, and their uptime exceeds that of any internal business
> email setup I've ever seen or helped run. You lose Outlook based
> calendar functionality, but you gain document sharing and
> collaboration to replace emailing bulky email documents around. And it
> plays very, very well with Linux clients such as Scientific Linux and
> even cell phones, unlike the Exchange suite. The spam filtering is
> also *ery* good.
>
> Unless you've got some very large demands for customized internal
> services or security far beyond that of most small shops, don't burn
> your time on setting up your own messaging or collaborative suite.
> Between managing high reliability services, backups, denial-of-service
> attacks, system security, customization requests, and migrating users
> to new tool suites,  you'll burn up any benefits from having it in
> house with months, if not within minutes, of first running it in house
> in a small environment.
>
> If you *have* to continue with Outlook based clients, especially for
> calendaring, look at Microsoft's "Online365" services.
>
> I'm afraid I can't recommend the locally installed, Linux based,
> "messaging suite" replacements for Exchange. I tried Zimbra under
> RHEL/CentOS some years back, and rejected it as too bulky and far too
> expensive in engineering time to maintain. I hope it's gotten better
> since, but it suffered from the same problem as Exchange: awkward
> integration of conflicting components and their requirements. I don't
> expect the mentioned "Zarafa" tool suite to do it any better, but I'd
> be curious to see more recent experience with either.
>
>               Nico Kadel-Garcia <[log in to unmask]>