I followed the install docs and installed Ubuntu 8.04 lts with Zimbra 6.0 and have no troubles. Server is a public mail server behind a router and firewall using 1to1 nat on router and port filtering by IP on the firewall. All DNS entries on both public and internal return as they are supposed to. All mail sent internally from users and externally from yahoo, msn, aol, gmail etc... all arrive correctly and send correctly.
It is great in every way except when I send certain pieces of email to the server from another server thoses pieces of email are not delivered into their boxes for the user to login and see via email. However the account forwards a copy to another account called archive@ and the email ends up in the archiver.
These few emails ones sent from an automated system are not being delivered into our inbox for various people. They get lots of other mail just not these. The mail that is not being delivered I see in the zimbra log file as being delivered. We even have copies going into an archive account and those copier are delivered. In the zimbra log file I can see the emails stating that they are delivered. We have turned off the anti virus and the spam assassin on zimbra and this still behaves the same way.