Postfix: Recipient address rejected: User unknown in virtual mailbox table

[joximu]

if you create the domain cws-drummer.biz on a ispcp system, then this system also feels responsible for mails for this domain.

And you have the webmaster/postmaster & abuse addresses - so I think this is the problem.

You may need to remove all cws-drummer.biz entries in you postfix/ispcp/* files and postmap them. Also do the changes in the working copy.