The Winmail.dat file is used to preserve Rich Text formatting. Outlook uses it when sending a Rich Text-formatted message. During transport, the content of the message may be changed, preventing the receiving client from being able to read the formatting instructions. In other cases, the receiving client does not use or recognize the winmail.dat file.
What is Wimmail.dat?
Free Online Service
Letter Opener Pro - http://www.creativeinaustria.com/LetterOpenerPro
is integrated right into Mac OS X Mail and Microsoft Outlook 2011 and converts these files seamlessly in the background.
Letter Opener Lite - http://www.creativeinaustria.com/LetterOpenerLite
is a standalone App to convert winmail.dat.