For only the list members to be able to send a message to the list (send-by-subscribers):

(a) The setting at Membership Management > Membership List > Additional Member Tasks > Set everyone's moderation bit, including those members not currently visible is selected as OFF.
(b) The setting at Privacy options > Sender filters > Member filters > By default, should new list member postings be moderated? is selected as NO.
(c) The setting at Privacy options > Sender filters > Non-member filters > Action to take for postings from non-members for which no explicit action is defined. is selected as HOLD.