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.
For more information, see https://support.microsoft.com/en-us/help/290809/.
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.