Several sites on the Internet subscribe to mailing lists, archive the messages, and make them publicly available. This may not be desirable, as the this often exposes the addresses of the subscribers. The subscribers, while happy to discuss their work on a peer list, may not appreciate messages from the general public asking for follow-ups to messages posted to the list.
ezmlm-idx lists, by default, are set up with a ‘X-No-Archive: yes’ header. For older lists or lists using ezmlm alone, just add this header to DIR/headeradd. Well-behaved archiving servers respect this. If you want to maintain full control, the only recourse is to make the list subscription moderated(*). Of course, the messages are no more “secure” than the least reliable subscriber. Remove the ‘X-No-Archive: yes’ header for DIR/headeradd if you want to allow well-behaved archivers to collect your messages.