Transport Neutral Encapsulation Format (TNEF) is a message encapsulation format used by Microsoft Exchange/Outlook. Usually this format is packed as an email attachment named Winmail.dat and Outlook Express hides this attachment automatically. See http://support.microsoft.com/kb/241538/en-us.
If the Messaging Security Agent archives this kind of email, and the extension of the file is changed to .EML, Outlook Express will only display the body of the email message.