Version: 1.2.9 (using KDE 3.5.9) Installed from: Gentoo Packages When I reply to a message, KMail inserts the appropriate headers (Refrences and In-Reply-To). After sending, when I go to the sent-folder, select the message and resend the message (don't know exactly how it's spelled in english, the second entry in context menu, after all the forward methods), a composer window opens and I can send the message a second time. But that message does not have In-Reply-To or References header fields and so cannot be inserted into threaded display at the recipients's side. As the second message is still a reply if the original message had been a reply, this should be considered a bug.
I can confirm this bug on kmail 1.11.1 on kde 4.2.1.
Works fine here on KDE 4.9.3 Please double-check and close in case it has been fixed.
Yes, with Kubuntu 12.10 live media, I can not reproduce the bug. It seems to be fixed without being noticed here.