List Flag: filtered

Many unwary users send requests to unsubscribe from a list to the list itself
rather than to the listmanager.  If this flag is set, listmanager intercepts
mail addressed to the list or to the list owners, attempts to determine if
the message body consists mostly of commands, and then interprets the
commands rather than distributing an unsubscribe message to the list or
the list owners.  Mail which cannot be identified as such continues on
to its intended destination.
