4.59. Why is the Mailman mail-to-USENET-news gateway munging the Message-Id: header?
There are two parts to this answer.
First, the RFCs require that the Message-ID: header be globally unique throughout the world, and the only way that Mailman can be reasonably sure of this is to generate its own header that it should be able to guarantee will meet this requirement.
Secondly, Mailman uses the value in the Message-ID: header to determine whether or not it has seen this newsgroup posting before, and whether it should copy that message back to the mailing list. This is to try to avoid duplicate postings to the list and loops between multiple bi-directional mail/news gateway systems.
If your mail program or newsreader implements message threading correctly (see http://www.jwz.org/doc/threading.html), then it should be able to deal with these differences. If not, then you should complain to the people responsible for implementing and supporting your mail program or newsreader.
As far as Mailman is concerned, this is an old issue. The algorithm that Mailman uses today was developed years ago, to deal with the problems that were being created at the time by the mail programs, newsreaders, and other mail/news gateways. However, many broken programs remain in use today, and these problems haven't gone away.
This is not an ideal solution to the problem, but it does appear to cause the least overall confusion and disruption to the affected communities.
If you have a better solution, one that takes into account the problems that have been experienced in the past, please feel free to post your patch on the Mailman tracker at https://bugs.launchpad.net/mailman.
For more information, see the threads at http://mail.python.org/pipermail/mailman-users/2005-January/041884.html, and http://mail.python.org/pipermail/mailman-users/2001-November/015730.html, among others.
Converted from the Mailman FAQ Wizard
This is one of many Frequently Asked Questions.