How to turn off TNEF to prevent recipients getting WINMAIL.DAT attachments for Outlook (Office 365)

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"When messages are sent from Microsoft Outlook or Microsoft Exchange Server, there is a chance that the recipient will see the winmail.dat attachment if they don’t have their mail program set up to handle messages in the Microsoft Outlook Rich Text Format (TNEF). The Rich Text Format is different because it allows the display of different styles, fonts, and colors within the email. All of that Rich Text information is stored in the winmail.dat file, and Outlook occasionally attaches regular file attachments in the winmail.dat file as well."

1. Log in to your Office 365 Portal (portal.office.com) using your Admin credentials.
2. Click the Admin icon.
3. Expand the Admin sub-menu from the left-hand menu, and click Exchange.
4. In the Exchange admin center screen, click mail flow from the left-hand menu.
5. Click remote domains from the top menu. 
6. Configure a specific set of rules for the affected recipient domains by clicking the plus (+) icon above the NAME field. 
7. Give the configuration a suitable name for the rule you're adding or leave it as Default
8. Add the fully-qualified domain name for the recipient that's having the problem with the winmail.dat attachments or leave it as *
9. Locate the "Use rich-text format" setting which by default is set to "Follow user settings" which you need to change to "Never".
10. Change the MIME character set to Unicode (UTF-8) instead of Western (ISO)
11. Click the Save button

https://www.techguidereview.com/prevent-sending-winmail-dat-attachments-when-using-office365-and-outlook-2013-2016/

Or

https://support.microsoft.com/en-us/help/2487954/how-to-specify-the-email-message-format-that-s-used-for-external-recip