Clean note not inserted in HTML POP emails

I’m using up to date WinXP SP3, OE6, IE7, Zone Alarm, SpyBot S&D (with TeaTimer) - oh, and this thing called Avast (free version).

My issue concerns emails received on POP accounts. I have Avast set up to insert a note into clean messages. All clean plain text only emails and all clean emails with attachments (regardless of whether plain text or HTML) have the appropriate clean note inserted. However, some (but not all) of the HTML emails without attachments that I receive do not have the clean note inserted.

From reading earlier posts, I’d got an idea that it might be to do with the senders creating the HTML messages as Content-type: multipart/alternative (as opposed to text/plain, multipart/mixed or multipart/related). But I have found cases where such emails do have the clean note.

So, I suppose the questions are:

  1. What, specifically, triggers the failure to insert the clean note?

  2. Is there anything that one can do to either ameliorate or completely resolve the situation?