Is it possible to set up Zimbra to accept mail for defined useres and forward mail for non-local users to another server?

Basically we would like to set up Zimbra and have the ability to migrate users one at a time to it. Users who are not set up on Zimbra would continue to receive their email as they always have in the past.

I know how to do this in a Sendmail installation. I believe there is a similar Postfix config file setting for this, but I'm not sure what it's named or where the Postfix config lives in Zimbra.