maildirman copies held messages from Mailman's moderator queues to a Maildir. After you move messages to "spam" and "non-spam" folders, it feeds them through SpamAssassin's Bayesian learner, and tells Mailman to discard or accept, as appropriate. This allows you to manage both list moderation and spamassassin training via a local MUA, a standard IMAP client, maildirsync, or any number of other standard mail-handling mechanisms, rather than only through the Mailman web interface.
These details are provided for information only. No information here is legal advice and should not be used as such.