Problem – you receive an email that has an attachment named winmail.dat that your Mac Mail program can't view.
Reason – The Winmail.dat file is used in Outlook when sending a Rich Text-formatted message however OS/X Mail as the receiving client does not use or recognize the winmail.dat file format.
Solutions
Ask the sender of the email to change their default email settings. Microsoft suggests 4 methods here. Then have the sender re-send the attachment. This is often impractical as it places the burden on the sender who may be someone you don't want to burden. It also only the solves your problem with this one sender and not the hundreds of millions other of Outlook users.
There are well established technical standards for email and so it may seem unfair that you are stuck with this problem because Microsoft chose to use a proprietary format. Life isn't fair.
It is faster and more practical to install an add-on to view winmail.dat files on a Mac. I use TNEF's Enough written by Josh Jacob.
Download the latest version, open the dmg file and drag the program into your applications folder. If you receive the occasional winmail.dat attachment, save it to your desktop, open TNEFF's Enough and select FILE -> OPEN, double click on the attachment listed in the TNEFF's Enough program window and select a save location. If you receive winmail.dat files often, drag the TNEFF app into your dock then drag the winmail.dat file from your email and drop it onto the TNEFF icon in your dock.