Hey Tobias,
that was partially what I was looking for - thank you for your help.
Regarding that and having controlled: Yes, my special Group is watching (notification level 3) that special meant category. And they all receive an e-mail upon a new posting in that category; and noone else does. So that is really fine.
But … when I post something in another category they all (all members of that special Group) get notification e-mails also. And that is not what I want. They have no entries concerning that Group in category_users.
Is it that caused by the mailing list mode?