So what you've done is install Zimbra on another CentOS5 server inside the LAN, is that correct? You've also made sure that the Firewall and SElinux are disabled on this server? Let's start with the obvious questions, what is the specification of this server? Do you have any errors in the Zimbra install logs? What do the foloowing show:
Code:
host `hostname`
dig yourdomain.com mx
dig yourdomain.com any
Post the details here and the DNS configuration files you used. When yopu post that information could you wrap each piece in 'Code' (just highlight a piece of text and hit the # symbol above) as it makes it easier to read.