Messages sent from older versions of Microsoft email apps such as Outlook or Microsoft Exchange might include an attachment with winmail.dat in the name. The file doesn't appear in the sending app, but it does appear in Mail. In other email apps, it might appear as a MIME section named application/ms-tnef. This happens when the sending app uses Transport Neutral Encapsulation Format (TNEF).
How to open a winmail.dat file on your Mac
The winmail.dat file might not contain any useful information, or it might contain a document or other file.
- Open the message in Mail.
- Choose File > Save Attachments to save the winmail.dat file.
- Open the file with a third-party winmail.dat viewer. You can find such viewers on the Mac App Store.