Office 365 Outlook Sending PDF files which appear as Winmail.dat on Mac computers

Posted on 2016-09-28
Medium Priority
Last Modified: 2016-10-05
User says they just installed an update to Office 365
Mac recipients are now getting  winmail.dat attachments

I know how to fix this on the Mac side, but it's hard to tell all the recipients to download and add software.

Outlook is connected to an Exchange Server.  Since the files are appearing as Winmail.dat, I assume that the version of the Exchange Server is such that it produces this output.  I don't have control of the Exchange Server, and don't know how fast they can upgrade or otherwise change the Exchange Server.

Is there anything that we can do on the sending side?
Is the problem caused by the sending of multiple PDF attachments?  Would sending one at a time help?

Question by:computerlarry
LVL 44

Accepted Solution

Adam Brown earned 2000 total points
ID: 41820386
The problem is caused by Exchange using a feature called TNEF encapsulation. TNEF is a formatting system that really only Outlook understands, so POP3/IMAP and other non-Outlook mail clients generally can't read messages that use it. What you'll want to do is disable TNEF, either for the specific user you're sending messages to, to the email domain your sending to, or to all domains. https://support.microsoft.com/en-us/kb/2487954 has instructions. Note that this has to be done in remote PowerShell for Exchange online (If you're using Exchange online as your Exchange server. If not, use powershell on the On-prem server). There is a link in that KB that will take you to another KB article that will explain out to connect to Exchange Online powershell.

Author Comment

ID: 41820488
Is there a setting in Outlook that can change the format from Rich Text to HTML?

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