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.