This is more of a how-to then a question.
This is an excellent step-by-step procedure for installing the open source version of Zimbra but there is a missing step or at least I found a bug.
Zimbra Collaboration Suite (ZCS) Open Source Edition v5.0 On Debian Etch | HowtoForge - Linux Howtos and Tutorials
Once you complete the how-to you'll test sending mail.
If you see errors in /var/log/zimbra.log like this
you'll need to do the following.
FAILED: Unix utility file(1) not available
apt-get install file
Find PATH and add /usr/share/file so it should look like this.
I don't know if you have to do this but I did.
Mail started moving after I added that.
su - zimbra