Are people receiving emails from your Office 365 users that are blank with an attachment called WINMAIL.DAT? It’s probably time to disable TNEF across your Office 365 organisation.

How to fix the Office 365 WINMAIL.DAT attachment issue

  1. Connect to Exchange Online via Powershell. Follow our quick guide here to get connected.
  2. Run this cmdlet:
    Set-RemoteDomain Default -TNEFEnabled $false
  3. You’re done!

What causes it?

This issue occurs when the receiver’s email client is unable to interpret a message sent from Microsoft Outlook in the Rich Text format. When you send an email from Outlook using rich text, a plain text copy of the email is sent, along with an attachment called WINMAIL.DAT. This attachment contains all the rich text formatting, elements and other data unique to rich text messages.

The problem is, a lot of email programs can’t open emails sent using this method, known as Transport Neutral Encapsulation Format – or TNEF. In order to resolve it, we need to force Exchange Online to convert rich text messages to HTML before sending them. We do this by setting the RemoteDomain property TNEFEnabled to false on the Default policy. See Microsoft’s article here for more info.

Here’s what it looks like before and after.

Default RemoteDomain policy before:

Fix Winmail.dat By Setting Remote Domain Policy

Default RemoteDomain policy after:

Remote Domain Policy TNEFEnabled Is False

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