This article focuses on how to prevent Winmail.dat file attachment messages from being sent through Outlook.
Procedure
Choose File> Options> Mail, in the Compose messages area, select HTML or Plain Text for Compose message in this format.
Click Internet Format and select Convert to HTML format or Convert to Plain Text format.
Edit the registry. Locate the registry key:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences(The version number of Outlook 2013 is 15.0.)
3.1 Choose Edit > New > DWORD Value.
3.2 Enter DisableTNEF and press Enter.
3.3 Right-click DisableTNEF and click Modify.
3.4 Enter 1 in the Value data field and click OK.
3.5 Choose File > Exit to exit the registry editor.
Note:
When using Outlook to send emails for a small number of accounts, the preceding settings will take effect. However, when sending email to many accounts that exceed the threshold, the above settings may not take effect or will send winmail.dat attachments.