Disable TNEF using the Registry Editor

If all the above method fail, you can resort to entirely remove TNEF by altering your registry settings. It will be a rather harsh way of doing that, and I only suggest it to System Administrator who know what they’re doing. Take in mind that perform this task will also disable each Outlook TNEF-based feature, such as the built-in Voting System and Meeting Manager.

• Close Outlook
• Launch the Registry Editor from the Start menu by selecting Run, then typing regedit and pressing ENTER.
• navigate through the following registry key:
  • Outlook 2007:  HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
  • Outlook 2010:  HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
  • Outlook 2013:  HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences
  • Outlook 2016:  HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences
• Add a new DWORD entry with the following name: DisableTNEF, and give it a value of 1.
• Close the Registry Editor.
• Restart Outlook.