FormatPeek

What is a Winmail.dat file?

Winmail.dat is a Microsoft Outlook attachment that stores rich-text email information and original attachments in the TNEF format.

Claire Bennett, FormatPeek editorial persona
FormatPeek Editorial TeamPresented by Claire Bennett, editorial personaPublished September 10, 2026 · Updated September 10, 2026
Private by designYour Winmail.dat file is decoded locally in this browser. It is never uploaded to our server.
Example result2 extracted attachments
winmail.datDecoded locally · Not uploaded

Example attachments

A safe Winmail.dat example with two harmless attachments.

Extracted attachments (2)

Why does Outlook create Winmail.dat?

Outlook and Exchange sometimes send messages using Transport Neutral Encapsulation Format. Outlook understands the package automatically, but Gmail, Apple Mail and other clients may expose it as a file named Winmail.dat, Win.dat or ATT00001.dat.

How to open it

  1. Save the Winmail.dat attachment from your email.
  2. Drop it into the viewer above.
  3. Review and download the extracted files individually.

Is Winmail.dat a virus?

The format itself is not a virus, but its contents came from an email and can include unsafe attachments. Verify the sender and scan extracted files before opening them.

Can the sender prevent it?

The sender can use HTML or plain-text email instead of Outlook Rich Text and ask their Microsoft 365 administrator to review TNEF settings. The recipient cannot change how the original message was packaged.