Manually adding the headers is a thing of the past now, as there is an AppleScript to automatically add the Reply-To headers to *all* outgoing mail automatically. Here is the info from the original poster:
Try the Applescript called 'Add Headers to Outgoing Email'. In case you don't read the Read Me: THESE SCRIPTS ARE NOT SUPPORTED BY APPLE. USE AT YOUR OWN RISK.The AppleScript writes the default header to a Mail.app preferences file, so it will add the header until it is edited or removed entirely with the "remove" script, also thoughtfully provided by cricket.
- cricket
[Editor's note: cricket is a member of Apple's Mail development team who occasionally posts to the X4U mailing list. I assume (hope!) it would be relatively trivial to modify this script to also always insert the "BCC" field. I took a quick look at the source, but it's well beyond my limited AppleScript experience. Anyone got a quick modification to add BCC as well?]

