Outlook/Exchange - Winmail.dat

If you use ZixGateway from ZixCorp for encrypting e-mail you may have run across an issue where your recipients start receiving attachments as Winmail.dat files. This is likely to show up when the ZixGateway is used as a smart host for your local Exchange server. The real issue is Outlook/Exchange are sending the e-mails as Microsoft's proprietary Rich Text Format. In order to resolve this issue you'll need to check several places in Outlook as well as Exchange to verify the RTF is not being used.

In Outlook 2013/2016 check under the Outlook Options, Mail settings. The heading is "Message format" and has an option to convert to HTML format when sending RTF to internet recipients.


Additionally you might want to check your Exchange server settings to ensure the remote domain transport is not set to always use RTF. Under your Exchange organization select Hub Transport, then under the Remote Domains tab select your default domain and click properties. From there you should see the option for Exchange rich-text format which can be set to Always use, Never use, or Determined by individual user settings.


To disable TNEF from the registry use the following steps:

1. Close Outlook
2. Start Registry Editor (type regedit in the Start Search box or Start menu, Run command and press Enter)
3. Locate the following registry key:
   In Outlook 2016:
   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences
   In Outlook 2013:
   HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences
   In Outlook 2010:
   HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
   In Outlook 2007:
   HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
4. Add a new DWORD named DisableTNEF.
5. Double click on DisableTNEF and in the Value data box, type 1
6. Close the Registry editor and Restart Outlook

More helpful details can be found here.