for Microsoft 365

How do I prevent Winmail.dat attachment from being sent automatically?

The message format can be configured by each user in Microsoft Outlook or on the OWA interface. It can also be configured in the Exchange Online Admin Center for all users.

Note: The Admin setting will overwrite each user's individual setting.

■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 "Default" to edit the default filter.
　　Note: This will change the setting for all of the recipients regardless of the destination.
5)　For the "Use rich-text format", click on "Never".
6)　Click on "Save" to save the setting.

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.