Mailaddress from certain server gets blocked - Spam Config Problem?

[Kolab, User]

Hi Folks,

I am running an Server with froxlor default configuration.
If I send an testmail with this server to the kolab mailserver the 
message gets blocked:

Serverfehler: "550 5.1.1 <xxx>: Recipient address rejected: User unknown 
in virtual mailbox table"'

What might be the catch?

kind regards and thanks.