Outlook seems to need to have the content type set correctly in order for it to view a page as html with html tags.
You might be able to do this by adding the
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> tag, but your best bet is to check out
MIME::Lite and generate a multi-part mime document. That should enable you to get the fixed-width in proportional e-mail clients that you were looking for.