The winmail.dat file contains unreadable formatting information of the email. If you use Rich Text Format or HTML, Outlook will automatically attach a winmail.dat file to the message. When you compose an email in Microsoft Outlook, you can choose from three formats: Rich Text, HTML, and Plain Text. So if an email attachment on your device is labeled “Winmail.dat,” the sender composed the email using one of those two programs. The Winmail.dat Opener Tool TNEF’s EnoughĪ DAT file is short for “Winmail.dat” These files are created by Microsoft Outlook or Microsoft Exchange.Try Saving The File In A Different Format First.Here’s what it looks like before and after. See Microsoft’s article here for more info. We do this by setting the RemoteDomain property TNEFEnabled to false on the Default policy. In order to resolve it, we need to force Exchange Online to convert rich text messages to HTML before sending them. The problem is, a lot of email programs can’t open emails sent using this method, known as Transport Neutral Encapsulation Format – or TNEF. This attachment contains all the rich text formatting, elements and other data unique to rich text messages. 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 issue occurs when the receiver’s email client is unable to interpret a message sent from Microsoft Outlook in the Rich Text format. Set-RemoteDomain Default -TNEFEnabled $false Follow our quick guide here to get connected. Connect to Exchange Online via Powershell.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 time to disable TNEF across your Office 365 organisation.

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