The major ways spammers determine whether you've viewed their e-mail are:
(a) You click on a URL that has an identifying parameter on it
(b) The mail client displays an external image that has an identifying parameter on it (e.g. img src="http://biz.p0rnwhackers.net/?id=victim347823413" width=1 height=1)
I've been using KMail for a good long while (I *love* the Bounce feature) - and even in HTML mode, I've never seen it pull in external images. The only images it will pull in are inline (i.e. data for them is sent with the message). So, AFAIK, you're pretty safe in KMail in that case. Try sending yourself an HTML e-mail with an externally-linked image (off the top of my head, I have no idea what mail client you can do such a thing in
Mind you, I wish there was an option to, on a per e-mail basis, ask it to show external images. It makes my e-mails from MGM and a few other places I've subscribed to a little difficult to read.