How to stop emails from being sent as winmail.dat attachments – Exclaimer Knowledge Base

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:

Option using PowerShell

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

Option using the Exchange admin center

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.

NOTE: Some functionality within Outlook, such as Voting Buttons, Tasks and Contact cards, require TNEF functionality. To retain this functionality alongside Exclaimer Cloud, it is necessary to configure some additional Remote Domains Settings. For more information, see How to use Transport Neutral Encapsulation Format (TNEF) objects in Exclaimer Cloud?