Sometimes emails from Windows users contain an attachment called winmail.dat or win.dat which mail applications on the mac do not automatically decode. If you reply asking the sender what is in the attachment they will most often say that they’d not attached anything. This is because their Microsoft email application mostly uses the dat file to store formatting code for the email to make it look nice to other users of the same software. However it is also sometimes a wrapper for attachments the sender does intend you to be able to open.

Why Apple Mail does not automatically decode it I don’t know. The decoding software is open source. So, in the interests of harmony with Windows users, here is a link to what you need to decode the dat files, TNEF’s Enough:

For OS X 10.5 and above

For OS X 10.4

Thanks to Josh Jacob