I have a domain bau-recht.ch

Localy there are some postfix users like

aarau@bau-recht.ch and so on

The real mailserver is an external server and i get the with fetchmail.

There is another location in another town the use the same domain
bau-recht.ch an the get here mails by outlook.

Now when i have to send som mails to one of these users not in our own
office they can't be sent. I get an error like:

Dec 31 12:18:30 localhost postfix/qmgr[10864]: 3232EE1B1D:
from=<testuser@bau-recht.ch>, size=1824, nrcpt=1 (queue active)
Dec 31 12:18:30 localhost postfix/local[10895]: 3232EE1B1D:
to=<test@bau-recht.ch>, relay=local, delay=0, status=bounced (unknow
n user: "test")

How to tell postfix to send mails without local user to an external mail
host like any other mail like something@bluewin.ch ?

tnx