Each Newsletter should be attached to at least one list. In this example, we attached the Newsletter to two lists : "Updates & Infos" which contains 24903 users and "Downloaded AcyMailing" which contains 12426 Users. So, if you click on the send button, this Newsletter will be sent to those lists and AcyMailing will take care of the duplicates entries (users subscribed to both lists will only get one e-mail). Nevertheless, you can see a warning on the list "Downloaded AcyMailing" because this list is not published! AcyMailing does not send a Newsletter to an unpublished list, you have to publish it first.