Version: (using KDE KDE 3.4.2) Installed from: Debian testing/unstable Packages Normally, a plain reply uses the address in the Reply-To header and reply to sender uses the From address. I have a message that (mistakenly?) has both a Mail-Followup-To and a Reply-To header. A plain reply uses Mail-Followup-To (correct) but a reply to sender uses the Reply-To address. That can't be right. The message in question is this one: http://lists.debian.org/debian-devel-announce/2005/12/msg00009.html
On further consideration, using Mail-Followup-To over Reply-To in a reply is also wrong. I guess this functionality was invented in the context of mutt(?) where you have a "reply" and a "follow-up" key. I don't know how to map this to kmail; perhaps kmail is missing a "follow-up" functionality (being different from "reply to all" and "reply to mailing list"). Of course what kmail really needs is the ability to make the reply behavior entirely scriptable by the user. :-)
Information about the intent of the Mail-Followup-To header (which KMail probably doesn't follow): http://cr.yp.to/proto/replyto.html If you read this document then you'll see that "follow-up" is equivalent to "reply to all". KMail's behavior stems from the fact that a normal reply is treated as "reply to mailing-list" if you reply to a mailing-list message. Given that everybody and her dog want to have different reply behavior making it scriptable is indeed the best solution. :-)
BTW, if you want to send a reply to a specific address then please use the "Reply To" option from the context menu of the corresponding address.