If emails sent to:
mailman@HostName and mailman-owner@HostName returns back.

Do the following:
pico /etc/aliases

comment/delete these lines:
mailman: postmaster
mailman-owner: mailman

Save and try sending emails again.

Mailman should reply to you this time instead of the return error if you sent to mailman@HostName.

cPanel.net Support Ticket Number: