Here's something that I found: "If the Cc or Bcc lines appear in the message body, make sure you're separating header lines with a new line (\n) rather than a carriage return-new line (\r\n). That should come at the very end of the headers."
You might also want to try moving that header before or after others to see if it fixes things.
An even better solution would be to switch to phpMailer, which is supposedly a much, much better solution for sending mail through php.