outlook rule to convert incoming email to plain-text

I "solved" this same problem by assigning a mail rule to pick up only the specific problem email/s, and moves it to "Junk". In junk, all emails are converted to plain text.

Thus I read these problem emails in my junk folder, which is much better than the status quo of outlook "not responding" for a minute every time I accidentally preview the problem email.

EDIT: I also added a notification alert to the mail rule so that I wouldn't "miss" the email

If you are concerned about the display and do not really need to convert see here for instructions to "Read as Plain Text" http://support.microsoft.com/kb/831607

You can edit a message with VBA. According to the MSDN, if you set the _MailItem.BodyFormat property to olFormatPlain it will discard all formatting.

This article on using VBA as a more powerful alternative to the Outlook filtering rules should set you in the right direction.