🚀 go-pugleaf

Well, only Dave Crocker knows for sure, but I think this interpretation is
correct:

New message (reply)		Old message (being replied to)
------------------		------------------------------
To:				use Reply-To: if present,
				otherwise use From: if present,
				otherwise use Sender: if present
				otherwise use Return-Path: (gag)

cc:				use To: and cc: if present
(including a cc: is at the
user's option-- some people
prefer to omit the cc:)

For an example of the rules that MH uses, look at the standard "replcomps"
file distributed with MH.  This is a "simple" formatting facility which
builds the reply draft for the user.

Note that the answer to your "have CC's get replies if I use REPLY-TO" query
depends on the recipient of the message, NOT the sender.

/mtr