Scenario
After sending an email, the recipient reports that they could not see the contents of the email and that there is an attachment named winmail.dat.
Reason
Microsoft uses a proprietary email format called Transport Neutral Encapsulation Format (TNEF) which works well in Microsoft email clients. Still, most other email clients, like Gmail, Yahoo etc., don't support TNEF.
When other email clients receive a TNEF email, they cannot display it properly and show the email content as an attachment called winmail.dat.
When an email leaves a Microsoft 365 environment, it should be automatically converted to a standard email (EML) format to avoid this behaviour.
Resolution
To stop emails from being sent as winmail.dat attachments, you need to configure your default Remote Domains Settings to not use TNEF.
You can do any one of the options described below - please click on the required options listed below to go through the detailed description:
- Connect to Exchange Online Powershell.
For more information, see how to connect to Exchange Online Powershell. - Run this command:
Set-RemoteDomain -Identity default -TNEFenabled $false
- Navigate to Exchange Admin.
- Sign in with your global administrator credentials.
- Select the Remote domain named Default.
- Click Edit text and character set.
- For Use rich-text format, select Never.
- Click Save.