Malte,
Let's think of the best name for the switch.
We'll need 1) a parameter to acs-mail-lite to specify if ALL multiple recipient emails go out this way (if its possible)
2) A switch to acs_mail_list::send (or complex_send, I don't know the exact difference) to specify the exact behavior for any individual email that would override the paramter. I'd add this because an individual application, ie: bug-tracker or other workflow based system, might want to use the CC feature, but for forums, you might not. It gets pretty complicated quickly.
For the mail tracking I would add a "to_type" column that could be "to" "cc" or "bcc". That makes the most sense to me, then you can default to "to" and it works the same as it does now, but if extended to support the new feature.
As long as everything is optional and the default behavior is the same as exists now it should work.
