Go to
Split DNS - Zimbra :: Wiki to see how to create a split dns.
From what I've seen, zimbra's mta, postfix, tries to authenticate itself and if the output of "host -t mx simonpope.co.uk" shows your external fw address from the zimbra server, zimbra will not route mail properly. If your unconfortable manually editing dns files do a search on how to use webmin to help. This was my major problem and once we added a split dns server to the zimbra box that only zimbra uses it's now working like a charm hosting several of our live domains.
Good luck.