Mac App Store Link: https://itunes.apple.com/us/app/winmail.dat-file-viewer/id1009148234?mt=12
iOS App Store Link: https://itunes.apple.com/us/app/winmail.dat-viewer-letter/id994423211?ls=1&mt=8
Winmail.dat File Viewer is the easiest, fastest, most reliable way to extract, view, and save the the sender, receiver, subject, and message text of winmail.dat files.
It's completely frustration-free: just double-click the win mail.dat file and you're instantly presented with a list of its contents. Double-click the attached files to open them, or drag and drop them on the desktop. In most circumstances, you won't even need to leave your Mail program.