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.
The problem: Some email recipients on Mac OS X using Apple Mail and Gmail receive winmail.dat attachments in place of correctly-encoded MIME attachments from users running Outlook 2016/Windows 10/Office 365 hosted mail. They can't open the faulty attachments and (in our case) the result is grumpy clients.
The cause: The issue is that Exchange can still encode attachments as RTF rather than MIME. There's no need for this in a hosted SaaS email service in 2016 - it's a legacy piece of Exchange functionality that's probably kept around in the code-base for reasons known only to Microsoft. But it's definitely a pain in the neck for those of us who don't need backwards-compatibility to dinosaur-era Exchange servers.
The solution: You'll undoubtedly come across various articles and suggestions that you need to indulge in a whole bunch of PowerShell jiu jitsu in order to fix the problem. In our case this didn't work - or at least not for any length of time. Thankfully, there is an (apparent) fix lurking in the Office 365 Admin GUI. It goes like this:
- Login to your Office 365 Portal (portal.office.com) using your Admin credentials.
- Click the Admin icon.
- Flip down the Admin sub-menu from the left-hand menu, and click Exchange. This will take you to your hosted Exchange admin page.
- Click Mail Flow from the left hand menu.
- Click Remote Domains from the top menu line. This will allow you to configure a specific set of rules for the offending recipient domains.
- Highlight the Default * domain entry, and click the Edit Pencil above the name field.
- About two-thirds of the way down the page, you'll find the "use rich text" setting - it will default to "Follow user settings". Change this to "Never".
- For extra credit, if you want to, you can change the default MIME encodings to Unicode (UTF-8) instead of Western (ISO).
- Hit the Save button.