Did you receive an email attachment named winmail.dat or winmail? Here’s a brief explanation of what this file is and instructions for opening it.

Winmail.dat files are sent by people using Microsoft Outlook as their email application. Under certain circumstances Outlook will encode attached files, like Word or PDF files, in this winmail.dat format.

Update May 2012: If you need to open winmail.dat files on your iOS device (iPhone, iPad or iPod Touch) try Winmail File Viewer which currently costs 99 cents in the iTunes Store.

Update February 2017: In November 2015, an iOS version of TNEF Enough was released by the developer of TNEF Enough.

Mac users can open the winmail.dat file using a free application (donations accepted) and extract the files hiding inside. Unfortunately, this application has a goofy name which is TNEF’s Enough.

Here are instructions on how to open the winmail.dat file:

• Save your winmail.dat file on your Desktop.
• If you use Mac OS X 10.6 or later then download the most current update of TNEF’s Enough version 3. If you use Mac OS X 10.5 then download TNEF’s Enough ver 2.2. If you use OS X 10.4 or earlier then download TNEF’s Enough ver 2.0.
• By clicking on one of these links, you’ll download a .zip file which contains TNEF’s Enough inside. Your Mac should automatically open the .zip file so you should see TNEF’s Enough sitting in your Downloads folder.
• Move TNEF Enough into your Applications folder.
• Double-click it to open, then go to the File menu and select Open…
• Select the winmail.dat file which you previously put on the Desktop.
• You’ll then see a window listing the embedded files which are the files that you actually want.
• Click on each file, one at a time, and click the Save button.
• Save these files to your Desktop or any other desired location.
• Quit TNEF’s Enough.

You’re done. You can now throw away the winmail.dat file since you’ve extracted all of the files from inside it.

If you’d like to read more about how and why this happens please read this Microsoft Support article or this other article. These articles also provide instructions that you can pass on to the Outlook user who sent you the winmail.dat file so he or she can make changes to no longer send you winmail.dat files. Essentially, they have to edit their record for you in their Contact list so it indicates that you should only receive plain-text emails.