listhelper is an antispam system developed for GNU mailing lists managed by Mailman. In short, it uses SpamAssassin, invoked via procmail, and some specific mailman configuration settings. Human volunteers review and correct real messages wrongly identified as spam and vice versa, along with approving real messages.
The system is somewhat more fully described on a Savannah wiki page. We've also written an exhaustive list of suggestions and comments about Mailman settings in general based on our experiences.
Listhelper is enabled for all new mailing lists created through the Savannah interface (Mailing lists -> Configure from a project page). You can disable it by removing the obvious address from the moderator field in the mailman configuration; then neither the listhelper software nor the human volunteers will do anything with incoming messages to the list.
After some years of experience, we realized that listhelper has problems when most of the real mail to a list is not in English. This is because the Bayes classifier is a large part of the spam/nonspam determination, and it doesn't have enough real mail in other languages. Although there's no way to explicitly determine the language which is mostly used on a given list, we figured that if the mailman interface language (the preferred_language value) is not English, most likely the mail to the list was not English either.
So, around 1 June 2013, we removed listhelper from all lists which had a different interface language, and will continue to do so with new lists as they arise. We know this makes for extra work for those list moderators, which we regret, but too much real mail was being deleted. We also set max_days_to_hold to 300 for these lists, since we knew many of them were unattended, and did not want spam to build up literally forever. 10 months should be enough to handle incoming messages.
This whole issue could be ameliorated with sufficient work; if anyone is interested in pursuing it, contact us. (And this section will be moved back onto the wiki page when the wiki is functional again. Sigh.)
There are two mailing lists related to listhelper itself. First, firstname.lastname@example.org is for general questions, discussion, bug reports, etc. The main system implementors (Bob Proulx and Karl Berry) are on this list, and other current volunteers. Second, email@example.com is for volunteers willing to handle the mailman pending queues for a large set of GNU lists.
Savannah project page, source tree (including README).