I don't have a direct answer as to why sendmail with a CC might not work - but that's a very, very old way of sending mails. The first thing I would try, because it's easy to do, is replacing it with a modern module such as Email::Stuffer. If you then still have the same problem, it might be a configuration issue on the side of the mailer - but even in that case, using the aforementioned module makes things easy, since you can simply switch to a different transport by using a different Email::Sender implementation.
