If you use Outlook 2003 with a POP or IMAP account,
you can tell Outlook to store the entire MIME source of the item when downloading.
The MIME can then be viewed via the View | Options menu on the item (the full
MIME will be located in the "Internet Headers" field).
To enable this feature, set the following registry
Data Value: 0x00000001
But what about on old mail? This only work on newly downloaded mail after the flag has been set.