Thanx for the reply. Unfortunately I think I totally don't understand what you are trying to explain, which, by the way, is not your fault but probably my lack of knowledge and experience
What I still don't understand is, why ISPConfig shows email@example.com
as the (default) e-mail address of the user in the sub domain, while I would expect to see firstname.lastname@example.org
, which is also what I see in /etc/postfix/virtusertable. That's what's confusing me.
By the way, I deliberately did not create a co-domain and followed one of your topics about creating a sub domain by creating a new site in the main domain. Why would I want to "create a co-domain with empty hostname"?