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.

If I sent a mail from the froxlor server e.g. to an gmx account 
everything works fine.
i think it might be an misconfiguration of kolab/spamassassin? I use the 
default spamassasin config.
how to check why the mail gets blocked?

kind regards.