Virtualmin - Zimbra - split domain


For a domain I have a Zimbra server as the primary e-mail server but some accounts stay and live on Virtualmin. I followed the split domain setup from Zimbra and I have everything working except one thing.
The delivery to accounts of the same domain that live on Zimbra.

This also means that I can not use the Relay module/plugin since not all email for the domain is relayed, only the unknown (local) once.

So I’m wondering if some one can point to or assist me in configuring Postfix mail relay for unknown local recipients?
I tried reallocated mapping but I keep getting “User unknown in virtual alias table” when I try to send from the Usermin account to an existing Zimbra account of the same domain. Other domains work correct.

Thanks in advance,