If you’re reading this, chances are you just received an e-mail message with a strange winmail.dat attachment file. Don’t worry, it’s not a virus neither a dangerous content: it’s just a container of data (files, metadata, info, notes etc.) built by s Microsoft Outlook client using the Redmond company’s proprietary data encapsulation format protocol (TNEF, as per Transport-Neutral Encapsulation Format). If you’re receiving that it means that the sender is most likely using either MS Outlook or some other Microsoft-based client system of the MS Exchange family (SharePoint, etc.).
If you don’t, in order to see what’s inside the aforementioned file you need to use some specific software such ash:
Both software are pretty much straightforward to use: download, install, and you’ll be able to unpack any winmail.dat file just like any other archive (zip, rar etc).
If you’re using Mozilla Thunderbird you can achieve even better results by downloading and installing the LookOut+ add-on by Attila K. Mergl, built as a replacement of the almost-identical (and now discontinued) LookOut by Aron Rubin: once installed, the component will automatically show the winmail.dat content (if any) just below your e-mail messages.
In case you’re the one using Outlook and you don’t want to send your attachments wrapped into a winmail.dat file, you can turn off the whole TNEF feature as explained in this guide.