What MTA are you using on your relay server ? If you are using Postfix you could either 1) perform a direct LDAP lookup on your Production server for the recipient and then relay the email to your Production server 2) Extract the list of users from your Production server using zmprov and then generate a transport file or 3) anything destined for the domain handled by your Production server relay it through.
If your using Exim have a read of
Exporting all addresses - Zimbra :: Wiki.