Getting an email from a Windows/Outlook user can come with a problem for those of us on Macs. Winmail.dat is a proprietary email file format generated by Outlook or a Microsoft Exchange Server that provides rich-text formatting and the problem is that the files can only be read by certain email clients. So unless you’re using Outlook or Express you might be out of luck.
The are a couple ways to solve this though. The first, and hardest, is to ask your Windows friends to stop sending them. Of course they probably don’t even know what they are so just get them to go into Outlook on Windows, navigate to Options > Tools > Mail Format. From the Send In This Message Format list, select either Plain Text or HTML, then click OK to save the new setting. Pretty big pain in the ass if you’re getting more than a few message with .dat.
The other way is to download a free program called TNEF’s Enough. It will allow you to open those pesky files hassle free. Just keep the icon in your dock and when you get an email with the winmail.dat file attached you can just drag it onto the icon to read it. Very quick and very simple.
If you come across something else please let me know.