I'm really stuck here, hopefully someone can help..

How can I force Zimbra to always use the MTA instead of deliver email locally for preconfigured domains in Zimbra.

I'm using GMAIL in this example:

I have setup a gmail.com domain in ZIMBRA, for my local email.
And also specified the SMTP server of my Internet provider as MTA.

But now when I sent email to another Gmail email user, the zimbra servers won't deliver this email to the MTA, but tries to deliver it locally. But the other user does not exist locally and then the email gets cancelled.

Sending email to other valid email adresses (not a local domain in zimbra) through the MTA goes fine.

zimbra.log

NOQUEUE: filter: RCPT from: <....>: Sender address triggers FILTER smtp-amavis:[127.0.0.1]:10026; from=<....> to=<....> proto=ESMTP helo=<....>
NOQUEUE: reject: RCPT from : 550 5.1.1 <...>: Recipient address rejected: gmail.com; from=<....> to=<...> proto=ESMTP helo=<.....>