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