Overview

The Transport Neutral Encapsulation Format (TNEF) is a proprietary email format used by Microsoft Exchange and clients, which includes Office 365. While sending between Microsoft based clients normally works without issues, when sending to a non-Microsoft based email client or an older email client, the recipient may receive a “winmail.dat” file instead of the attachment or where there’s formatting in the email.

Note

The best fix for this is to set Outlook to use HTML as it’s outgoing format and this should be set before using this article.

This article will take you through the steps to disable TNEF for a specific domain where all other mitigation within the sending client has failed.

Tip

You will require Global Administrative rights for your Office 365 account to access this.

Instructions

  1. From within your Microsoft 365 admin center, go to the Exchange Admin:
    how to disable tnef in office 365 for a specific domain
    Hint: click “Show all” if you can’t see it in the left hand menu
  2. Go to Mail flow -> Remote domains :
    how to disable tnef in office 365 for a specific domain
  3. Add in a new remote domain:
    how to disable tnef in office 365 for a specific domain
  4. Set a name for the remote domain, then specify the domain:
    how to disable tnef in office 365 for a specific domain
  5. Leave “Email reply types” settings as-is and click Next.
  6. Leave “Message reporting” settings as-is and click Next.
  7. Within the “Text and character set”, set Rich Text Format to Never:
    how to disable tnef in office 365 for a specific domain
  8. Set the Supported Character set to Unicode (UTF-8) and click Next:
    how to disable tnef in office 365 for a specific domain
  9. Review the final settings then hit save.
  10. Test by resending an email to the recipient receiving the winmail.dat files to ensure they’re now receiving the emails as expected.

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