In the New Mail or other folder window:
Right click the message, and select Message Properties.
In the right hand column uncheck the box beside Contains HTML data.
Click OK. That should allow you to see the message as a text message only.
Click Ctrl-H to bring up the full headers.
Highlight the HTML spam in the new mail folder
Open a new email message
Drag the HTML spam onto the new message
In the dialog that appears select "Show All Headers"
Highlight the entire message, then copy to clipboard
Paste message into the form.