I had a look at an older email, which had these headers...

Restarted mailman and disabled -> enabled the RFC 2369 option.

Let's see...

On Mon, 2012-09-17 at 22:00 +0200, Stefan G. Weichinger wrote:
> Am 17.09.2012 21:58, schrieb Michael Tremer:
> > Sorry for the delay.
> > 
> > On Wed, 2012-09-12 at 20:21 +0200, Stefan G. Weichinger wrote:
> >> ps: regarding this mailing list:
> >>
> >> Why do the headers of the mailing-list-mails not contain a "List-Id" ? I
> >> use that for server-based filtering with other lists. Very handy ...
> > 
> > I don't know. Does mailman support "List-Id"? RFC-2369 headers are
> > enabled, so I guess that will include List-Id.
> 
> I don't see/find it (even in your latest mail now).
> But I can also filter for "Organization: IPFire.org", no problem.
> 
> Thanks, Stefan