actually, your post was sent both in plain text AND HTML. while there is no clear way to force plain text in OWA prior to using MS Exchange Server 2007, the lbo admin can easily set Mailman to automatically convert these multipart alternative formats so as to forward only the plain text on to the list.
i posted on this a few days ago, but to repeat:
1. go to lbo-talk administrative interface |----> Content filtering,
2. set filter_content to Yes
3. add these to pass_mime_types: multipart/mixed multipart/alternative text/plain
4. and set convert_html_to_plaintext to Yes.
We've used this on Marxmail for several years to successfully remove HTML-formatting altogether, and this will convert messages such as Stephen's to pure plain text.
Les Schaffer