Could it be I found a bug? I modified an entry / domain in ispconfig to use an external mailserver (ispsite / options / external mailserver). Unfortunately, the entry in the postfix configuration file /etc/postfix/local-host-names is not removed. Even worse, when modifying the second domain, the 1st domain returns in local-host-names. I have to remove these entries manual (and restart/reload postfix). I expect by using an external mail server, the local-host-names would not contain these domains?