Hi,
on the firewall, you should forward port 80 (http) and/or port 443 (https) from the external IP to the IP of the Zimbra server, depending on how your users access Zimbra, though HTTPS is
*strongly* recomended if you're setting up internet connection, for this you have to tell zimbra to listen on https.
This is called DNAT, virtual server or some kind of forwarding in the firewall/routing interface on the router.
Browse some doc, or ask if you require more info
Best regards.