New Email Server

We plan to replace our email server this year and I feel like we are at a crossroads. We currently use Microsoft Exchange for our email server and it is on version 2003 sp2. We also have the licensing to upgrade to 2007 with software assurance. While we are completely satisfied with Exchange I still feel compelled to take a look at some other solutions.
Some of the available solutions that have beckoned me...

1. Zimbra - Seems to be a fairly flexible message and collaboration suite that was just recently purchased by Yahoo!. It was written with open source but there is a version available that has a support option. The native interface is a cross browser compatible ajax enabled web client but you can use Outlook and most other common email clients including those for Apple or linux. It can run on Unix or Linux. It integrates with Active Direcory or other LDAP enable directories.
2. Go completely open source with mail and server. I am no where daring or knowledgeable enough to implement this but for some reason I'm always drawn to the idea. But with the importance of email these days it needs to be a guaranteed.
3. Go totally with a hosted solution. The options for hosted solutions are almost endless. You can have a hosted Exchange, Zimbra, Sendmail, or anything in between. This gives the responsibility to someone else for disaster recovery. Google mail looks the most cost effective and the integration of other google apps it increases the functionality. Hmmm, very tempting.

Sad to say that after it is all said and done I'll most likely stay with MS Exchange. Why you ask? Well, it is what everyone is used to. We are almost exclusively a MS shop so why stop now. The price point is good. I'll keep you updated on the new install of my exchange server. I plan to do a migration from 2003 to 2007.