From: DeskNow Knowledge Base
http://www.desknow.com/kb/

I receive an email with a winmail.dat attachment. How do I open it?
http://www.desknow.com/kb/idx/4/097/article/

winmail.dat is a proprietary Microsoft format used by old versions of Outlook. It can typically be read only by Outlook. Please see this page:

http://www.pchell.com/support/winmaildat.shtml

 

Normally, modern versions of Outlook do not use this format.