[exim] Routing mail to different internal server

My company is migrating from Sendmail to Zimbra. We use Exim 4 as our
smtp gateway server.

We plan to run both mail servers in parallel for a short period while
we make the transition.

We have a few test users that will start using Zimbra immediately. For
these test users, I want the Exim gateway
server to route incoming mail to the Zimbra server.

How do I make this happen?

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Regards,
Conrad Lawes