I was recently trying unsuccessfully to setup mailman to provide list features for some groups of friends. My problem was that instead of the classic usage of aliases, I was trying to follow the newer ‘HOWTO’ provided by the debian folks. For some reason the mail would never go through, except one copy to me. At first I figured that was because I was the list owner possibly, but that was incorrect. As it turns out, I had a specific feature added that was causing my problems.
You see, Exim router definitions (in a split config) are loaded in order, and take that precedence in said order. Well, I like using wildcard addresses, so I had a 950_unknown_user definition sitting in router/, and was adding a 970_mailman* like the documentation gives and expecting it to work. A little renumbering, and vioala, things work as they should. The spiffyness of this automated config is very cool, and I now can recommend it over the old method of using aliases.