Cc and Bcc are not the same.  If you don't want your recipients to see the
addresses of the others who also receive the messsage, you must use Bcc
(View>Bcc FIeld).  Then you have a couple of choices.  You can simply leave
the To and Cc fields empty or you can put your own address in the To field
and the DL name in the Bcc field.  You can also create another contact whose
DIsplay As field matches that of the DL (or any other phrase you want, like
"Undisclosed Recipients", give it your address, and put that in the To field
while putting the DL in the Bcc field.  Finally, you can skip the DL
altogether, put the addresses of the contacts in a separate Contacts folder,
and use mail merge to send an individual message to each contact in that
folder.