Exchange server replacement that runs on Linux

I've worked as a sysadmin for some years and what I keep coming back to is that users like Microsoft Outlook and want to use its Exchange features. I have tried my fair share of commercial alternatives but usually there is either a fundamental feature missing or there are stability issues.

In short I am looking for a Microsoft Exchange Alternative with the following features:

• Authentication through SQL or LDAP
• Has a solid, comfortable web interface for the users when they are off-site
• Supports replication and load balancing (if one fails, the second one should be already running)
• Outlook client support (or a really good alternative client)
• Resource booking (meeting rooms, projectors, company jet, etc)
• Calendar (shared/private) and Email (if that wasn't obvious)
• (Optional) A cross-platform client for us *nix users.
• (Optional) Corporate support contracts available
• (Optional) An open-source software is a plus

Please keep your answers as detailed as possible to determine that you've successfully deployed the software and it fulfills the needs. If I wanted a list of claimed alternatives , I would simply Google it.

I've personally tried Binary Server, Novell Groupwise, homegrown Postfix/Cyrus stuff and in the end the 'real thing' because those users just love Exchange.

Please help me find a good alternative.

Zimbra is an excellent opensource, linux based alternative to Exchange. It combines Apache Tomcat, Postfix, MySQL, OpenLDAP and Lucene in a single, well defined package. It offers:

• LDAP Authentication
• Calendaring, resource booking and free/busy info
• Ability to connect using outlook, and its own Zimbra client
• Excellent web interface
• Allows multi server setup and replication

It is available for free, with the option of paying for a supported version. I have used Zimbra for a number of organisations who did not, or could not pay for Exchange, and they have all been very happy with it.

At my last job I migrated us off of Intermedia's Exchange service to Kerio Mail Server. It has great webmail (much easier and lighter than OWA), LDAP, ActiveSync and support for NotifyLink/Sync. It looks like Exchange to anyone using Outlook or Entourage. GAL, resource booking, calendars, built-in spam and AV.

Stores everything in flat RFC822 files so no broke Exchange store, has a great management interface and stellar support.

It really does most anything Exchange does, but is more light weight, doesn't require hughe hardware or licensing costs. I loved it while I was on it.