Winmail.dat replaces attachments sent by Office 365

What is RTF, TNEF and Winmail.dat?

Outlook can use a special method, technically referred to as Transport Neutral Encapsulation Format (TNEF), to package information for sending messages. The use of TNEF is affected by settings in Outlook that are referred to as Rich Text Format (RTF). TNEF and RTF are not identical, but they are very similar.

A TNEF-encoded message contains a plain text version of the message and a binary attachment that "packages" various other parts of the original message. In most cases, the binary attachment is named Winmail.dat, and it includes the following information (if included in the message):

• The formatted text version of the message (ie, font and colours).
• OLE objects (such as embedded pictures and embedded Office documents).
• Special Outlook features (I.e., custom forms, voting buttons, and meeting requests).
• Regular file attachments that were attached to the original message.

Where to check for RTF settings in Outlook

When RTF is not working as expected (either all messages are RTF, or Meeting Requests and Voting are not working) there are several settings your need to check in Outlook.

Note: Exchange server administrators can also control RTF to Internet addresses. If the administrator disables RTF to Internet addresses, the settings in Outlook will not override them.

Global Properties

Composing new messages:

Outlook 2010, 2013, or Outlook 2016: File, Options, Mail and at the top. Set “Compose messages in this format” to HTML.

Use this setting to control how messages created using RTF formatting or that require TNEF encoding are handled. If this is set to plain text or HTML formatting, Voting and Meeting Requests may not work unless you override the setting using Email properties.

Outlook 2010, 2013, or Outlook 2016: File, Options, Mail and scroll to the bottom of the dialog and set “When sending messages in Rich Text Format to Internet recipients” to “Convert to HTML format”.

Recommended setting: Convert to HTML. Outlook will send all messages to the Internet using HTML unless you change the email properties (below).

Outlook 2007 and older: Go to Tools, Options, Email Format, Internet Options button.

You can also change the settings on a per-contact level by opening the contact card and double-clicking on the email address.

Registry Setting

If nothing seems to be working and you use Outlook 2007 SP2 or newer, you can use a registry value to end TNEF encoding once and for all. This will affect your ability to use features that require TNEF encoding, including Voting and Meeting Requests in native Outlook format.

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:
1. In Outlook 2016:
   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences
2. In Outlook 2013:
   HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences
3. In Outlook 2010:
   HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
4. 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