listhelper is an antispam system developed for GNU mailing lists managed by Mailman. In short, it relies some configuration settings and filters mail through conservatively-configured SpamAssassin, bogofilter, and crm114 processes. Human volunteers review and correct real messages wrongly identified as spam.
Entirely independently of deleting spam, if desired, human volunteers can also approve real messages. The listhelper software never does approvals itself.
The system is somewhat more fully described on a Savannah wiki page. We've also written a comprehensive 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, as a project administrator). 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, and you will be entirely responsible for deleting spam. If you wish to keep the automatic spam removal, but not the human approvals, please contact us.
There are two mailing lists related to listhelper itself. First, email@example.com 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, firstname.lastname@example.org is for volunteers willing to handle the mailman pending queues for a large set of GNU lists.
Savannah project page, source tree (including README).