Update:
First of all, we're still evaluating Zimbra. We are not in production yet.
We designated one computer to serve as the test Zimbra server. This computer is sitting in the LAN and has a public IP, a public A record and a public MX record.
All records are resolvable from the internet and from the LAN and firewall configuration has been made to allow the Zimbra server to send and receive mail to and from the inside.
We installed a DNS server in the Zimbra test computer (bind9) and added the copmuter's internal IP address in an A record. We also added an internal MX record.
Installation runs fine, but we are unable to send mail to the outside.
This is the message we see in the mail queue:
ECB_Error.jpg