Stop Outlook from Sending the Winmail.dat File as an Email Attachment
The cause of the mysterious Winmail.dat file attachment when sending email via Outlook is a mysterious one. We know it’s not harmful, but the problem has existed since Outlook 2007, and Microsoft hasn’t taken any steps to actually get rid of it.
At this point it’s really more of an annoyance than anything, but it’s an annoyance you can get rid of in a few simple steps.
1. Exit Outlook and type regedit in the Search (or Cortana) box. Click the program to continue.
2. Allow the app to make changes by pressing the Yes button.
3. Click HKEY_CURRENT_USER and then navigate to the appropriate folder: Software > Microsoft > Office > (highest version, 16 in this case) > Outlook > Preferences.
4. From the Edit menu, click New and then DWORD Value.
5. Type DisableTNEF and then right click and select Modify.
6. Type 1 for the Value data and then click OK.
7. Exit the registry editor and restart the computer.