You have something already on port 25?
Did you do a fresh install after changing hostnames? (follow the
UnInstall Zimbra - ZimbraWiki directions first)
What's your /etc/hosts look like?
I'm really supprised you don't have a /opt/zimbra/log/mailbox.log though...
"not owned by root: /opt/zimbra/postfix-2.2.9/conf/main.cf"
as root run the fix permissions script:
cd /opt/zimbra/libexec/
./zmfixperms