Are your recipients receiving emails from your Office 365 users that are blank with an attachment called WINMAIL.DAT?
How to fix the Office 365 WINMAIL.DAT attachment issue
- Connect to Exchange Online via Powershell – instructions here
- Run this cmdlet: Set-RemoteDomain Default -TNEFEnabled $false
- You’re Done
What causes it?
This issue occurs when the recipients email client is unable to interpret the message sent from Microsoft Outlook in the Rich Text format. When the message sent from Outlook uses 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 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). To resolve it, we need to force Exchange Online to convert rich text messages to HTML before sending them.
This is done by setting the RemoteDomain property TNEFEnabled to false on the Default policy.
See Microsoft’s article here for more info.