We support a small office that uses Office 365, hosted Exchange and the Office 2016 desktop software (including Outlook). When they send PDF attachments to people outside the organization some recipients receive a Winmail.dat attachment instead of the PDF attachment.
Here are a few articles that describe the problem and some fixes. But these fixes didn't seem to work for our setup. It seems the hosted exchange isn't honoring the request of the client and continues to send in Rich Text format.
Using the latest configuration from Microsoft, why is this still a problem? I found these steps in a Social Technet Article to disable the Rich Text format from the O365 portal.