We have patched this issue. (Attached)
Do not replace Message-ID, just add X-Mailman-ID
Skip message if X-Mailman-ID is set
That skips the rewriting of Headers and keeps threading for Mailinglists as well as Newsgroups intact.
a minor fix for stupid MUAs in gate_news:
del 'Newsgroups' header, because MUAs like Evolution take the first useful occurance of a valid Header line to reply. For messages coming from NNTP that is the 'Newsgroups' header. But having that header in a Message received via Mailinglist, leads to a reply towards the newsgroup, even if there a valid To: and/or Reply-To: headers set. Replying to a Newsgroup from a Mailaccount breaks usabilty (user might not even have a nntp account).