Disabled TNEF and messages with PDF still arrive as winmail.dat


Hello, some of our customers and our users using Office 365 start see after last builds Office 2016 strange things about sending e-mails with PDF attachments.


Outside world complain, that they receive winmail.dat instead of original e-mail. I know, there are programs how to read this type of attachment and recover message but, it is not good for users and experience.


What is strange in this issue, is that at Office 365 side, the TNEF format is disabled (set to "Never") in all Remote Domains. Also registry key to disable TNEF on Outlook 2016 side are set correctly. All name cache was deleted and records in address book check for settings. All of them are correct.


But other side still receive winmail.dat. The combination, which we tracked are:


  • Sender: Outlook 2016 with PDF attachment, at Windows 10 from Office 365
  • Recipient: Outlook, every version, iOS and macOS and Windows, on Office 365 or different mail server

Do you see some other things to look at?

2 Replies

If you've done it at the tenant level then it *should* be working.

This article might help, but you've probably already done it all: http://www.deargeek.co.uk/2013/11/how-to-stop-office-365-sending-those-pesky-winmail-dat-attachments...


Otherwise I'd suggest creating a Service Request to see if perhaps the TNEF setting isn't actually being applied.

Thanks to reply, Loryan.


Yes, we did that on tenant level in Exchange. We also full patch all Outlook 2016 clients and check their settings and they seems to be correct at contact or application level.


We now try to create explicit Mail Contacts to such problematic external users with TNEF disabled, maybe it helps. We also created Service Request to official support, but no progress so I also wrote here for sure.