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]>