Your filter (or filter module, if you decide to use one of the modular frameworks) would need to check the list of envelope recipients (the headers aren't exactly authoritative as far as actual message delivery is concerned) for the copy address, and then either return an "accept" or "reject" result. [...] It should be possible to add the copy address to the list of envelope recipients from within a courierfilter. This can be achieved by opening the control file in append mode and appending an additional "r" control record[4] with the copy address. However, I'm not entirely sure this would actually cause Courier to deliver the message to the added copy address. (Perhaps someone on this mailing list knows better than I do?)