One of our sites is set to "External Mailserver", but is still listed by domain name in /etc/postfix/local-host-names. If I remove this domain from that file, and restart postfix, then mail will be delivered to the domain outside of the server (as it should be). My issue is that the domain is somehow put back into the local-host-names file and mail is trying to deliver to localhost. I noticed that it's "put back" later in the day after I removed the listing. I have tried to switch the perference in ISPConfig to "Local Mailserver". This will list the domain name in the local-host-names file, but if I switch it back to "External Mailserver", it's still listed in the file.
This sould be the solution to your problem. The main domain of a website and the co-domain are independantly listed in the local-host-names configuration and you must set them both to external mailserver.
