Welcome to the forums.
Zimbra needs valid A & MX records in order to be able to deliver mail, pick any one of your domains e.g. domainonline.com set-up the correct records and modify your hosts file for the LAN IP and you should be OK to send and receive email fro all your domains. I have exactly that set-up for a couple of domains and only one is listed in my local DNS server (both domains in external DNS) and both domains send & receive OK.
[edit]In fact you hosts file format should be something like this :
Code:
192.168.x.x mail.domainonline.com mail
where mail is the hostname and mail.domain.com is the FQDN of your server.