Hi
Welcome to the forums.
Yoo can't ignore this error, you'll need to address it I'm afraid.
If you intend to have a mail server visible to the world and if you're going to have your DNS records hosted outside your LAN then you'll need the correct ports forwarded through the router and you'll need a local DNS server set-up as 'split DNS'.
If you have a domain name then it will point to your IP address and each of the PCs on the LAN will need a 'name' (they probably have already but let's say 'mail' for the zimbra PC), the zimbra would then be mail.domainname.com
You'll need to check that your /etc/hosts file is set-up correctly and the hostname for the zimbra PC can be resolved via DNS. When you've done all that you should install Zimbra, that should see you right.

In (very) simple terms, that's all there is to it.
Having said all that if you're just testing zimbra and you don't have a registered domain name and the server won't be accepting any connections from the internet then it's slight more simple to setup. You'll still need a hostname for the zimbra PC and you'll still need a locan DNS server. You can find a DNS 'how to'
here and apart from that there isn't much to it.