Winmail.dat files are attachments sent using Microsoft Outlook. This type of file can be difficult to open on your Mac Computer. Fortunately there is a FREE APP that you can use for these files which will open them as a pdf. I use this FREE App and haven’t had any issues. It’s name; TNEF’s Enough.
How To Use The APP
When you open the email consisting the attachment, Right Click and choose the TNEF’s Enough application to open the file. It will ask you where to save the file, I choose Desktop, then you can open the file with Adobe Acrobat Reader DC.