Sometime the List-Post is missing (for historical reasons, or because the mbox files are not the archives of a mailing list). This patch parses the list's address from a $DIR/.listname file when the List-Post is missing.
Created attachment 28654 [details] patch, v1
committed with some tweaks as r1331072, thanks
I think the precedence is wrong here. If the .listname file is present, it should be used in preference to List-Post, rather than as a fallback. This is necessary for use with archives that are not mailing lists but which may nevertheless contain posts that also sent to mailing lists. If the .listname does not take precedence, then the info that is extracted will vary depending on whether the current mbox has a mail with a List-Post or not.