Alright - finally I got the users. mailboxes and addresses back - most likely because of a config-restore without reconfiguring network - although the process failed. Anyways - now I am at this status, where everything seems to be working excapt sending emails to the world. Internal emails are deliverd properly but no emails are sent to external addresses - they still stuck in the queue.
Could it have something to do with the special network config in a VPS environment? I think about that dummy0 interface and that maybe the route the email has to take in the nethservers services is not correct?
I guess it is better to ask a new question and mark this one here as resolved - I got the network configuration done so the task asked in the title is solved.
regards
Sehne