Version:           1.8.1 (using KDE 3.4.1, Gentoo)
Compiler:          gcc version 3.4.4 (Gentoo 3.4.4, ssp-3.4.4-1.0, pie-8.7.8)
OS:                Linux (ppc) release 2.6.11-gentoo-r6

It would be nice if kmail filters could understand if a mail is a reply to one of my previous mail or not.
The test, not 100% reliable, but working in most cases, could be something along the lines of

for every email "sentEmail" in sent-mail folder; do
  if curMsg.subject().inReplyTo(sentEmail) return true;
end
return false;