Thare is no such bug in ispconfig, just a misconfiguration on your sever. You can not define a domain of a external mail provider as local domain on your server and you dont need that to fetch emails. Thats not ispconfig specific, if you tell a mail server that it is authoritive for a domain name then it catches all email addresses of that domain an rejects unknown addresses. So you told the server that you are the owner of wanadoo.fr and the server acts like this.
Thre is no need for such a configuration, if you want to ftch emails from a existing wanadoo.fr account, then use the ftchmail function to get the emails from the pop3 or imap account.