If you use the Feature Assistant extension, there is enhanced support for BCC - and checking the “Include Youth Protection Notice in youth messages” option will place a notice in the email at the bottom, just before the name list if it was included, as shown in the example below:
Scouts! In keeping with Youth Protection’s no one-on-one contact rules, please remember to CC your parents when replying to this message.
Adults! In keeping with Youth Protection’s no one-on-one contact rules, please remember to CC any Scout’s parents if you include a Scout in a reply.
Whenever BCC is checked, it includes recipents this way:
This message was sent to : Gary Bearchaser, Colin W., Tony Wiggles, Sarah Wiggles
Additionally it always copies you on the messages you send.