Thanks to everyone for their excellent posts! Name resolution was my problem as well.
I was getting the same error and used this doc to address it. I ended up doing:
Zimbra 5.0.6 on Ubuntu 6.06
- split DNS
- hosts file
I'm embarrassed to say that I'd mis-keyed my hosts file and entered the IP address of the VMWare host which Zimbra was running inside. My habitual ping test came back positive. Don't let that be you!