when the mail server is out of the box

[ephigenie]

the Problem is the domains map - postfix assumes he's responsible for the domain as long as there's an entry for it in the map. (/etc/postfix/ispcp/domains* )

It doesn't matter if you've no mailboxes defined.
Solution : remove the unwanted domains out of the domains file, ran postmap against the file and put it below the working dir, too so that the next run of ispcp-rqst-mngr doesn't revert your changes.