How to prevent Winmail.dat attachment being sent automatically?

For Office 365

 

Q.

How to prevent Winmail.dat attachment being sent automatically?

 

A. 
Winmail.dat is a Rich-Text format that is automatically sent as an attachment along with the E-mail.
You can change the email format to a text format or HTML format to avoid Winmail.dat being sent. 

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

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

 

 

■User Setting(OWA)

◯Changing the Default Setting
 1) Click on the SnapCrab_NoName_2018-5-21_16-41-41_No-00.png icon on the top right, and click on "Mail".
 2) Under the "Layout" options, click on the "Message Format".
 3) In the screen, next to "Compose messages in this format:" and change it to "Plain text", lastly click on "Save".

◯Changing setting for each E-mail.
 1) On the top where …  appears, click on "Switch to Plain text".
 2) In cases where any messages are written in the email body, there may be a window that says "Some of the features...",
and 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 the "Options" option.
 2) Click on the "Mail" in the menu.
 3) In the "Compose Messages" option, "Compose messages in this format:", choose "Plain Text".
 4) Lastly, click on "OK" to finish.

◯Changing setting for each E-mail.

 1) On the top, choose the "Format Text" and select "Plain Text".
 2) In cases where any messages are written in the email body, there may be a window that says "Some of the features..." ,
and click on "Continue" to confirm.

 

■Administrator Setting

1) A Global Administrator that holds a account to login to "Office365 Admin center".
2) On the left menu, go to "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" to edit the default filter.
  Note: This will change the setting for all of the recipients regardless of destination.
5) For the "Use rich-text format", click on "Never".
6) Click on "Save" to save the setting.

Screen_Shot_2018-11-23_at_11.53.49__1_.png 

It may be possible to do via the command-line, 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.

          
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