Announcement Only List

Creating an Announcement Only list requires that posts from both members and non-members be disabled, leaving only moderators and list owners able to post.

These settings to control how incoming messages are handled are under "Message Acceptance". The settings for both members and non-members should be set to "Reject" or "Discard", and additionally the list moderator or owner needs to make sure the Moderation Action for all list members is set to "List Default" ("Reject" or "Discard" would also work).

Additionally, under "Settings" and "Alter Messages" are some settings that are recommended for announcement lists.

As shown above, "Anonymous list" should be checked and "Include the list post header" unchecked. This will further discourage posting to the list and conceal the original sender address.