Recipients of Outlook messages receive Winmail.dat

There is a long standing “problem” between Outlook and Internet email – Outlook uses a proprietary email format that only Outlook and Exchange (and a few other clients, including gmail) can decipher. When the recipient doesn’t use Outlook, they get a plain text message and an attachment named winmail.dat. Additionally, the attachment icon may be hidden by some clients, including Outlook Express and Windows Mail, making it look like the attachment was removed.

The problem? Any attachments included in the message are encased in the winmail.dat attachment.

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 colors).
• 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:

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

Outlook 2010, 2013, or Outlook 2016: File, Options, Mail and scroll to the bottom of the dialog.

Email properties of a Contact (Outlook 2007 and older):

Open the Contact and double click on the email address. Changes made here apply to all messages sent to this person. Use this to override the setting in Internet options (above).

Email properties of a Contact (Outlook 2010, 2013, or 2016):

In Outlook 2013 and 2016, look for Open Outlook Properties on the context menu when you right click on the email address in an open contact. Note that you need to be really fast and right click before the hovercard comes up. Once the hovercard comes up, the Properties link is missing from the context menu, often until you restart Outlook.

If all else fails…

If nothing seems to be working and you use Outlook 2007 SP2 or Outlook 2010 or 2013, 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.

PLEASE NOTE! FOLLOW THE STEPS BELOW AT YOUR OWN RISK. REGISTRY MODIFICATIONS SHOULD ONLY BE MADE BY A PROFESSIONAL. UNITED PC IS NOT RESPONSIBLE IF YOU MESS UP YOUR COMPUTER BY FOLLOWING THESE OR ANY STEPS FOUND ON OUR WEBSITE.

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 2007:

   HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences

   In Outlook 2010:

   HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences

   In Outlook 2013:

   HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\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