Having received another letter in the mail, the user may encounter a situation that some of the information in it is missing and there is winmail.dat in the file list. This file can be downloaded to a computer without any problems, but it cannot be opened by regular means. In this case, important information may be in the winmail.dat file, since, in fact, this file can be called a kind of archive with data. In this article, we will look at why winmail.dat may appear in the file list, and also how to open it better on your Mac.

Why .dat File Don’t Open Correctly on Mac?

Messages sent from older versions of Microsoft email applications, such as Outlook or Microsoft Exchange, may include an attachment with a winmail.dat in the name. This file is not displayed in the sending application, but is displayed in Mail. In other mailers, it may appear as a MIME section with the name application/ms-tnef. This happens when the sending application uses the TNEF format.

Read more: How to Solve Problems with the Work of the Mail App after Updating macOS?

Open .dat File on Mac

In most cases, DAT files are files that are incorrectly attached to e-mails; they are called “winmail.dat” or “ATT0001.dat”. Follow the steps below: 

  • Open the email with the attachment. In most cases, winmail.dat;
  • Click on the file and select Save Attachment;
  • Go to Download folder;
  • Right-click on .dat file and select Open with;
  • In the list, select TextEdit;
How to Open winmail.dat File on Mac?
  • In this program, you can open most text files, view the text and find out which program this DAT file works with.
How to Open winmail.dat File on Mac?

Note: If the .dat file does not open in TextEdit, most likely it is not a text file. Remember that some .dat files are locked and cannot be opened.

Winmail.dat Viewer for Mac

To view the files winmail.dat there are some applications for macOS. Open App Store and search for:

  • Winmail.dat Viewer;
  • Open Winmail.dat – File Opener.
How to Open winmail.dat File on Mac?

This apps could open winmail.dat files, also apps extremely simple and free.


Please note, I do not recommend downloading files from winmail.dat that are suspicious and are executable, that is, they have the format .exe, .cmd and others. In all other cases, as can be seen from this material, the problem with the .dat format is very easy to solve.

It’s the best time to ask questions and give us your feedback in comments.


Hi there! My Name is Vincent Lago! My goal is to share insightful reviews, guides, and manuals for people looking to know more about current solutions for Apple hardware and OS. I have years of technological background knowledge including owning a custom iMacs & PCs which was using for different types of business. And now, I want to share my experience with you.

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