I think, I have found a Bug, if I set on one of my Domains the Mailserver to "Externer Mailserver" ISPConfing adds the domain to /etc/postfix/local_host_names. With a very older Version of ISPConfig only the Domains addded which are set to "Internal Mailserver". Sorry, I found my mistake, in CoDomains I have also set to external Mailserver.
