Hmmm, that would be nice! Do we even know which configuration file that goes in?
Again, I may be way behind you on this, but I've found the following wiki article...
Split Domain - ZimbraWiki
Is that how you're planning on setting your Zimbra server up as a gateway? Using the terms from the wiki article your Zimbra server is primary, Exchange is secondary, and the main difference from the article being -
1. NO addresses are permanently stored on the Zimbra server, ALL are forwarded on the Exchange server, and
2. You want to check AD for verifying recipient addresses rather than adding them to a list or manually adding the accounts to the Zimbra server and setting each one up to relay to the Exchange server (which would be what we're discussing in this thread)
Is that correct?
Anyway, do keep me updated on how it works out for you if you don't mind.
Thanks!