Thanks for your Reply! This seems to be my exact problem.
But I have some questions about the solution.
Would it be enough if I changed the domain in the postfix config file from mydomain.net to mail.mydomain.net?
and another thing.. am I missing something if my main.cf has no $mydomain? Mine has only "myhostname" but without that $ sign. Does that matter?
And if that doesn't work at all, what can I do about it? The system runs on the domain of my Company and I use that domain as a virtual host. I can't see any other solution except running my companys website on a separate Server... do you?