Turns out, that mailinglist mode is working different for categories and group messages. In categories you gt even your own posts, in groups you don’t. Wich is sad, because so we can’t switch of mailman, there are a lot of the “older” users who used to manage their “Mailinglist” in their email client. Detailed talk about it, here:
Thanks a lot for still having a look into it, 'cause we still want to switch from mailman…