Roland Studer wrote:

Itis really strange, actually sometimes I got the posting with correct headers, while others got the very same posting without those headers.

In that case, this almost certainly has to be something occurring after Mailman has sent the message to the outgoing MTA, either in the outgoing MTA or further out in the delivery.

I don't see how it could be a Mailman problem.

Now this time I got the mail without headers, I haven't heard yet, if others got it the right way.

So it's only happening for a part of the subscribers and not always for the same ones. This is really puzzling.

Yes it is.

All I can suggest is to look at the Received: headers of the posts and try to determine if there is something involved in delivery of only the 'bad' posts.