A change as resulted in unexpected results. By opening up the archives of the mailing lists to all members it means that everyone can read messages sent between all email groups. If you have a mailing list for your Board and members of the Board have been discussing sensitive matters by email, those messages can be open to all you members to read.
My immediate response was to go in and delete sensitive messages, there is a better way.
Go into Permissions > MailingList Module and un-check access mailing list email for your Members
My preference is for only members of a particular mailing list to be able to read the archives of messages sent to a list.