For only the list administrator to be able to send messages to the list (send-by-owners):

(a) The option for Membership Management > Membership List > Additional Member Tasks > Set everyone's moderation bit, including those members not currently visible is set to ON.
(b) The option for Privacy options > Sender filters > Member filters > By default, should new list member postings be moderated? is set to YES.

IMPORTANT NOTE: For the messages with viruses or spam messages that appear to be coming from the moderator(s), as a measure you can make the below settings effective.

i. The e-mail addresses authorized to send messages to the list SHOULD NOT BE a member of the list. If the authorized address(es) belong to the moderator(s) the same is effective for them as well. If these address(es) is(are) member(s) of the list, their membership(s) is(are) cancelled.
ii. The e-mail address(es) to be authorized to send messages to the list should be entered at Privacy options >Sender filters > Non-member filters > List of non-member addresses whose postings will be immediately held for moderation as one e-mail address per line.
iii. The setting for Privacy options > Sender filters > Member filters > Action to take when a moderated member posts to the list. is selected as either "Reject" or "Discard".
vi. After these settings the e-mail address(es) authorized to send messages to the list can send to the list. The messages sent from these address(es) are submitted in electronic media to the attention of the list administrator and/or the moderator to be approved.
v. The list administrator with the administrator password, if assigned, moderator(s) with the moderator password, access the http://mailman.metu.edu.tr/mailman/admindb/LIST-NAME page approve the message(s) waiting for attention. Only then the message(s) is(are) distributed to the list.