What is the deal with winmail.dat files anyway?
The files are usually received by SMTP based e-mail programs from Microsoft Exchange and Microsoft Outlook users. The SMTP based e-mail program will usually either receive a MIME attachment named "winmail.dat" or a MIME attachment with the type "application/ms-tnef."
The file is a rich text (or MAPI) message that is sent from Outlook to Exchange. When Exchange sends the message to an outside server it writes the MAPI message as a MIME attachment. The unfortunate side effect of this plan is that if the Outlook user has someone in their address book as a person who can receive "Rich Text" then the user will receive the TNEF file whether the user uses Outlook or not.