Version: (using KDE KDE 3.4.2) Educate users about threads. So if the user replies to a thread but removes all quotations and makes up a new subject query him if he really wants to reply to other email or start a new conversation on its own. Tell him that thread hijacking is killing a nice feature of emailing. Also offer a link to some information what threads are anyway. Pressing the "Start new conversation" removes all references, in-reply-to etc. and sends the email as if it is a new one.
Nice idea. Don Sanders.
Thank you for your feature request. Kmail1 is currently unmaintained so we are closing all wishes. Please feel free to reopen a feature request for Kmail2 if it has not already been implemented. Thank you for your understanding.