How to fix the Office 365 WINMAIL.DAT attachment issue

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


  1. Connect to Exchange Online via Powershell.
  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 using 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 (TNEF). In order to mitigate this we need to force Exchange Online to convert any rich text email messages to HTML before sending them out. We do this by setting the RemoteDomain property TNEFEnabled to false on the Default policy. Further information can been seen in Microsoft’s article.