Winmail.dat Opener is the easiest, fastest, most reliable way to extract, view, and save the the sender, receiver, subject, and message text of winmail.dat files. Winmail.dat files are in fact TNEF format (Transport Neutral Encapsulation Format). Microsoft Outlook e-mail program for Windows send e-mails as TNEF. Most other e-mail programs. including Mail, do not understand TNEF. If your e-mail program doesn't understand TNEF, you may only see an attachment named 'winmail.dat' that you cannot open. The easy-to-use solution to circumvent this problem is provided by Winmail Reader. Note:This app only lets the user see what files are contained win the .dat file, and that actually previewing or saving the individual files requires a purchase.