Is the recipient address domain in /etc/postfix/local-host-names?
Does your mail actually arrive on your ISPConfig machine, or is it sent to another system because of some DNS misconfiguration? Have a look into your maillog (under /var/log) to see if the mail is at least arriving on your system.
You can run
tail -f /var/log/[your_maillog_file]
to watch real-time what happens when you try to send a mail to your ISPConfig system.