Background

Replacing Microsoft Exchange is very simple these days. The problem for most people is that they don't realize they will need several applications for it. Here is a guide which will help clarify things:

1. SMTP: First, you need to replace  the SMTP(mail send/receive) server. Set up a Linux server and install Postfix(if you're brave) or Sendmail or see a comparison of mail servers. We recommend Courier for ease of use.
2. MAPI: Courier supports this. It's the proprietary protocol used by Microsoft Outlook to communicate over the LAN to the server. You'd be better off switching to LDAP(address books and active directory integration), IMAP(client access to mail, better than POP3), and iCAL rather than fuss with MAPI. You'll ultimately benefit more, Outlook supports it, and you can use many other clients because they all support these protocols rather than MAPI.
3. Webmail: Courier has this.
4. Calendar: Courier has this, iCAL may be a little better if you have remote users sharing calendars and need permissions, meeting requests, tasks, etc. ICal integrates well with Outlook and completely replaces 90% of the functionality that Exchange provides.
5. Mobile Devices: SyncML will allow you to synchronize PIM(contacts, calendars, etc) from most mobile devices.The application which does all the work is called Funambol(yes it supports blackberry, iPhone, iPod, etc). Funambol also provides backup solutions and push-technology for mobile devices as a bonus.
6. Single-Sign-On: This comes from LDAP integration with Active Directory.
7. Anti-Spam: Courier has this with SpamAssassin, including blacklist support, user-defined lists, etc.
8. Anti-Virus: Courier has this as a clamav extension.
9. Encryption: Courier supports this with OpenSSL, including TLS.
10. Resource Booking: See Calendars above.
11. Out of Office AutoReply: Postfix does this and integrates nicely with Courier.

Hardware

We recommend a server with the following configurations:

1. Enterprise System(1000+ users): CentOS,  500GB+ RAID1 Drives, Dual-core 1GHz+, GigaEthernet(CAT6), 2GB RAM
2. Small Business(100-1000 users): Ubuntu LTS Server with LAMPP, 160GB+ drives, dual-core 800MHz+, 512MB RAM
3. Home(1-100 users): Ubuntu Server with LAMPP, 80GB+ drive, 800MHz+, 256MB RAM

Setup

1. First get your server up and running with SMTP. Verify it works with all clients you will need to support.
2. Next get webmail working. Verify with the browsers you need to support.
3. Get IMAP working and switch over to that. Send a few test emails, verify all clients support it.
4. Get LDAP working. Upload a few files to the server to test it.
5. Get iCal working, again verify.
6. Integrate LDAP with Active directory. Test by logging in with your windows credentials to the new mail server.
7. Finally get Funambol running and test all the mobile devices you need to support.

After all these pieces are in place, switch all of the clients over as follows:

1. Add the new server to their client
2. Copy all calendars, tasks, emails, contacts, etc over to the new server
3. Remove the old server

If all goes well, the users won't even notice other than you changing the server and copying the data over. If you really want to be sneaky, just log in as them when they're gone and do it all without their knowledge(not recommended). There may be other tools which can mass-migrate all of the user data but you'll need to find that on your own.

If you'd prefer to hire an expert, we're available and charge less than it would cost to buy licenses for Exchange or other  Open-Source systems such as Zimbra, Open Xchange, Scalix, Citadel, or OpenGroupware.

You can also do this yourself by following the instructions in our White Paper.