Originally Posted by pjdevries
I figured it out. I'm still a Linux newbie so it took me some time before inspecting /etc/postfix/virtusertable. In there I discovered that the actual e-mail address created for the user of the subdomain is not firstname.lastname@example.org, as ISPConfig shows on the ISP USer form, but email@example.com, just as one might expect. So the apparent problem turns out to be a small bug in ISPConfig.
No thats not a bug, thats the intended behaviour. Email addresses where created for the main domain and all co-domains of a website if you do not set a domain to external mailserver.
You just missed to create a co-domain with empty hostname for this website. This co-domain is created automatically when you did not set the domain limit too low.