I know this is a late reply but I am still having problems for some reason. I set external server in both locations. I looked in my local-host-names file and the domain in question is no longer in there. However in my mail.log file I am getting the following:
Jan 28 17:20:59 sp2979a postfix/smtp: E095D7549E0: to=<email@example.com>, relay=none, delay=0.06, delays=0.04/0.02/0/0, dsn=5.4.6, status=bounced (mail for mydomain.com loops back to myself)
I have restarted both ispconfig_server and postfix. I have stopped and restarted postfix. Is there anything else i need to do to make this work?