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[Email DLP] Configuration to Prevent Delivery of Winmail.dat (TNEF Format)

Target

  • Customers using Microsoft Outlook, Outlook on the web

Purpose

  • Configure settings to prevent the delivery of Winmail.dat (TNEF format).
    Check the user's Outlook settings or your Microsoft 365 tenant's settings, and disable the Winmail.dat (TNEF format) settings from the Administration if necessary.

Notes

  1. The content of this article is based on the product as of November 2025 and may change without notice thereafter.

Procedure

  1. Log in to the Microsoft 365 Admin Center with an account that has the Global Administrator role.
  2. Click "Admin Centers" - "Exchange" in the left menu to open the Exchange Admin Center.
  3. Click "Mail Flow" and then click "Remote Domains" from the opened screen.
  4. Click the Default row and then click "Edit Text and Character Set".
    This setting targets all emails, but you can specify any value according to your environment.
  5. Select "Never" for "Use Rich Text Format".
  6. Click "Save".

For command line configuration, please refer to the following site.
How to specify the email message format to prevent external recipients from receiving attachments as Winmail.dat (external link)
※ For inquiries about the details of "Winmail.dat", please contact Microsoft.

Frequently Asked Questions

What is Winmail.dat and why is it necessary to prevent it?

When an email is created in Microsoft Outlook Rich Text Format (TNEF format), it may be sent with an attachment called "Winmail.dat".
"Winmail.dat" is an attachment data that encapsulates the necessary body, layout information, and attachment information for the TNEF format.
In Email DLP, this "Winmail.dat" is treated as a regular attachment, making it subject to ZIP encryption and Secure Download.

If the recipient's client can read TNEF format emails, "Winmail.dat" will not be displayed, and the email will be received in the same state as sent by the sender.
However, if the recipient's client cannot read TNEF format, it will appear as if an unopenable "Winmail.dat" is attached.
Additionally, files attached by the sender are included within "Winmail.dat", resulting in only "Winmail.dat" being attached.
This leads to the phenomenon of files being attached that the sender does not remember sending.

Therefore, customers using Email DLP are requested to prevent the delivery of Winmail.dat (TNEF format).