If an e-mail contains only a HTML and a text part, the default message view shows a scrambled form, where the original line breaks and paragraph marks have been discarded and the HTML tags shown as part of the text.  General understanding in other e-mail programs is for the e-mail client (in this case claws) to choose one of the alternatives and show that alternative using the appropriate safe viewing code (so obviously don't use something that runs scripts or is otherwise insecure).