Split domain - Postfix - Zimbra

[jdp]

Hello,

For a domain I have a Zimbra server as the primary e-mail server but
some accounts stay and live on Virtualmin (Postfix mail server). 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.

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,

Johnny