The option mail_show_to_cc_list is installed with value "yes" but when do a manual notification and CC some people, the email recipient does not see CC list. The tech explains one possible reason for this unexpected behavior.
Release : 17.3
Component : SERVICE DESK MANAGER
When you use a custom notification method, this option can't guarantee the recipient will see the CC email list.
When do manual notification, choose the out of box email notification method.