I'm sure most of you know that Mailman is a major provider of backscatter. I've been trying to get the developers to pay attention to this issue for a number of years, and have provided patches to disable most of the backscatter, but those have largely been ignored. Queries, comments and suggestions on backscatter prevention were completely ignored.
A lot of abuse-admins mutually agreed it was time to start putting the screws to Mailman, and holding it to the same standards we hold every other mail application. I went on the list and brought this to their attention. For which I got a lot of flack I really wasn't in the mood to deal with, and dealt with poorly. But the only answer the actual Mailman developers gave was: we'll make it better in 2.2 (unknown timetable) and probably solve the problem for real in 3.0 (far future, not even a prototype yet)
So as of today, my employer and various others are explicitly banning un-patched Mailman installations in our networks. The first time we receive a backscatter complaint from a Mailman installation, we're going to give them standard resolution time (~24 hours for non-ongoing activity) to either patch Mailman to prevent backscatter, or disable Mailman entirely.
I highly encourage other abuse helpdesks to adopt the same policy.
Here's the good news. It had been a long time since I evaluated MLMs, and I was happily surprised. Not only are there a lot more choices on the market, but every free MLM that is currently supported deals with these problems better than Mailman.
In fact, the following two products already provide the kind of rules-based logic that Mailman developers "think might be possible in 3.0". If you're running a Mailman installation today, I highly encourage you to migrate to one of the following products (both freeware)
Dada Mail: http://mojo.skazat.com/
Sympa production can be configured properly, but the version in SVN appears to have much better defaults and logic. They are active and aware of the issues involved here. http://www.sympa.org/dev-manual/antispam