When I test new versions, I set up a sub domain and forward a copy of my email to the sub domain. In Zimbra on the sub domain, I set up my Persona so the email appears to come from
me@domain.com rather than
me@zimbra.domain.com", so responses will still go to the original domain, and then forward to the Zimbra server.
Obviously calendar invites, etc., will only work if they are sent to @zimbra.domain.com, but it will let you test things out with a copy of real email. You just won't get the full experience without mail going directly to Zimbra - but it works without interfering with your main setup too much.