Why is Outlook 2003 showing full headers in the message body area?

Why is Outlook 2003 showing full headers in the message body area?

I am having a strange problem with Outlook 2003.
It seems that random emails are not being parsed properly and Outlook
shows a blank sender and blank subject, and then the full headers in the
message body when it is opened. This only happens for some messages - some
display as expected.
We're running Outlook 2003 on Windows XP and POP3 download from Yahoo Mail
- although they display perfectly well in Yahoo before they are
downloaded.
Which settings should we be looking at to try to fix this? I can provide
screenshots on request.