I run only one zimbra server and use dns cnames for the the other domains. example
mail.zimbraserver.com CNAME zimbraserver.com
also i point new domains at the main zimbra server so the domain users do not have to remeber anything but there own domain.
mail.newdomain.com CNAME zimbraserver.com
mail.newdomain2.com CNAME zimbraserver.com
This works like a charm except for certs, there is always a warning about a name mismatch.example
your are attempting to connect with mail.newdomain.com but the cert is for zimbraserver.com this could be a security threat that may or may not ruin your/my life.