Comment 10 for bug 383784

I've verified this using the recipe you provided in your previous comment. I've also verified this using a stock Mailman 2.1.9 installation, so I believe it is a bug in Mailman. Launchpad uses the latest stable MM release (2.1.12) so I will try updating my test instance and see if the bug still exists, though I suspect it will.

There's definitely something client-dependent here. First, the forward should be as an attachment, however Claws (Linux) and Mail.app (OS X) seem to handle attachments differently because while they both fail, they fail in different ways. The Claws failure is closer to what I think you might see. I think Mailman is messing up the MIME structure of the message, and possibly breaking headers in the wrong place.

Launchpad's implementation of mailing lists isn't to blame here.