for Microsoft 365
Target
Applicable to customers using Microsoft Outlook and Outlook on the web.
Purpose
Configure settings to suppress Winmail.dat (TNEF format) delivery.
Check the user's Outlook settings or the settings of your Microsoft 365 tenant and disable Winmail.dat (TNEF format) settings if necessary.
Notes
The content of this article is based on the product as of January 2024 and is subject to change thereafter without notice.
Frequently Asked Questions
When an email is created in Microsoft Outlook Rich Text Format (TNEF format), it may contain an attachment called Winmail.dat.
Winmail.dat is an attachment that contains (encapsulates) the body, layout, and attachment information required for TNEF format.
HENNGE Email DLP treats Winmail.dat as a normal attachment, so it is subject to ZIP encryption and HENNGE Secure Download.
If the receiving client is a mailer that can read TNEF format mail, Winmail.dat will not be displayed and will arrive in the same state as the mail sent by the sender.
However, if the receiving client is not capable of reading TNEF format mail, the Winmail.dat will appear to be attached, which cannot be opened by the receiving client.
In addition, since the file attached by the sender is included in Winmail.dat, only Winmail.dat is attached.
This causes a phenomenon in which a file is attached that the sender does not remember sending.
For this reason, we ask HENNGE Email DLP users to suppress the delivery of Winmail.dat (TNEF format).
Detailed Procedure and Explanation
■User Setting（OWA）
◯Changing the Default Setting
1) Click on the icon on the top right, and click on "Mail".
2) Under the "Layout" option, click on "Message Format".
3) Change the parameter next to "Compose messages in this format:" and to "Plain text" and click on "Save".
◯Changing the setting for each email.
1) On the top where … appears, click on "Switch to Plain text".
2) In cases where any messages are written in the email body, a window that says "Some of the features..." may appear. Click on "OK" to confirm.
■User Setting（Microsoft Outlook 2016）
◯Changing the Default Setting
1) Click on the "File" option in the top menu, and click on "Options".
2) Click on "Mail" in the menu.
3) In the "Compose Messages" option, "Compose messages in this format:", choose "Plain Text".
4) Click on "OK" to finish.
◯Changing the setting for each email.
1) On the top, choose "Format Text" and select "Plain Text".
2) In cases where any messages are written in the email body, a window that says "Some of the features..." may appear. Click on "Continue" to confirm.
■Administrator Setting
1) Log in to the "Microsoft 365 Admin Center" on the Global Administrator account.
2) On the left menu, go to the "Administrator Center" and go to "Exchange" for the Exchange Online Admin Center.
3) Click on "Mail Flow" and click on "Remote Domain".
4) Double-click on the "Default" row.
Note: This setting targets all emails, but please specify any desired values according to your environment.
5) Click on "Edit Text and Character Sets."
6) Set "Use rich-text format" to "Never."
7) Click "Save."
It may be possible to configure the same settings using command-lines. Please refer to the following article.
How to specify the email message format that's used for external recipients to prevent Winmail.dat attachments
https://support.microsoft.com/en-us/help/2487954/how-to-specify-the-email-message-format-that-s-used-for-external-recip
※ For more details about "Winmail.dat", please refer to Microsoft Support.