Anytime I have started a new thread on this list, I have started out by replying to some other message (just to get the users@spamassassin.apache.org e-mail address in the To: field (deleting any others that might be in there), then I completely delete the subject, retype a new one, completely delete the text in the body of the message then type my message. This shouldn't break threading AFAIK. This is the same way I created this e-mail.
Yes, it does break threading. There are additional headers (References: for instance) which are used by many programs to determine threading. Those headers are created when you reply to a message, and remain after you delete the subject and text.
-- Kelson Vibber SpeedGate Communications <www.speed.net>