Postfix Problems with ISPCP

[YourDoom]

Hi,
I have a few users and they're using external MX records (external mail servers).
But when they send Mails to their own domain postfix throws an error:

Code:

### postfix/local[5551]: BA984704589: to=<admin@###.tld>, orig_to=<www-data>, relay=local, delay=0.24, delays=0.13/0/0/0.11, dsn=5.1.1, status=bounced (unknown user: "admin")

because the user and mailbox doesnt exists on this server.

how can I fix that?
I even don't get "log" mails, cause my primary mail address has a Hotmail MX record :/

kind regards