Outlook.com accounts: there is a "known issue" where Outlook.com accounts are sending winmail.dat files even though everything is set to use HTML or plain text. Microsoft is investigating. "Outlook.com Winmail.dat Bug" (Feb 2017)

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 OE 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.

RTF settings | Email properties of a Contact | Email properties in To field
Turn off RTF Capability | Tools | More Information

Note that if you are using Exchange Server, the Exchange administrators can disable RTF on messages sent to the Internet. SharePoint Contacts linked to Outlook are also a problem. See Other causes of unexpected formatting for more information.

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.

Don't forget to clear the auto-complete cache after changing Outlook's settings. The auto-complete cache (nickname cache) remembers the setting and Outlook will continue to send messages using RTF. Delete the address from the cache to clear it or clear the entire auto-complete cache. See How to clear name from the cache at Understanding Outlook’s Auto-Complete Cache if you need help clearing the autocomplete cache.

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 2010, 2013, or Outlook 2016/2019: File, Options, Mail and scroll to the bottom of the dialog.

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.

Email properties of a Contact

Note: this setting is not easily accessible in Outlook 2013 and newer.

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).

Recommended setting for most contacts: Let Outlook decide. This will send all messages using the global properties. If you need to send Meeting Requests or other messages that require TNEF encoding and know the contact uses Outlook, you can set the contact to always use RTF.

Email properties of an address in the To field of a new message:

Double click on the address in the To field. If the address is in your Contacts folder, their contact opens; if the address is not in your Contacts, the email properties dialog opens. Use this setting to "one-off" the formatting when sending meeting requests or RTF formatted messages to Internet addresses. This will override the setting in Options dialog's Internet options.

Recommended setting: Let Outlook decide - this will send all messages using the global properties. If you need to send Meeting Requests or other TNEF encoded messages and know the contact uses Outlook, you can set the address to always use RTF.

To access the Email properties in Outlook 2010 and newer

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 you set the LegacyGal registry value, you'll be able to double click on the address and open the properties dialog. You can also turn off the hovercard so the card comes up only when you click on the address. These ready-to-use registry files will enable the 'legacygal' feature and turn off the hover feature: Outlook 2016/2019/365 Outlook 2013
(You'll need to restart Outlook after setting the key.)

For more information about these settings, see Use Outlook's Contacts, not Contact Cards.

In Outlook 2010 and newer you'll access the email properties from the contact card that comes up when you hover over or click on an address in a Contact or an email message. Select the menu icon on the right and then Outlook properties to open the Email Properties dialog. As with the older versions, the recommended setting is Let Outlook decide unless you need to force RTF.

Reminder: The autocomplete cache (nickname cache) may remember the RTF setting and Outlook will continue to send messages using RTF, overriding the Contact settings. Delete the address from the cache to clear it. (Select the address using the arrow keys and press Del)

Other causes of unexpected RTF formatting

SharePoint Contact libraries synced with Outlook are marked to always use RTF. This is fixed in Outlook 2010 SP1 or the Exchange administrator can configure Remote Domain transports for the domains that don't use Outlook and set them to never use RTF to get around the problem.

To change the settings on Exchange server, open the Exchange Management Console, expand Organizational Configuration, Hub Transport, Remote Domains, open default transport (or the transport you want to change, if you have more than one) and change the Rich Text setting.

See Configuring TNEF settings on Exchange Server for the PowerShell cmdlets to use and instructions for configuring it in Office 365 Exchange.

Antivirus scanning corrupts the TNEF attachment and the recipients Outlook can't decode it. This is more common with client side scanners, such as AVG. To fix, disable email scanning on the client side; there is no need to scan email with a virus scanner on the desktop computer.

As mentioned previously, the nickname cache can contain the 'always use RTF' flag - if the sender ever forces RTF for one message to the contact, the cache remembers the setting for that contact and always uses RTF (forcing it on the address overrides the conversion setting in Mail format>Internet), even though the contact is no longer set to always use RTF. Delete the address from the cache to fix.

If all else fails...

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. (Ready-to-use .reg files are available below.)

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 2016/2019/365:

   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

Do It For Me

If you don't want to make the changes to the registry yourself, you can download a .reg file with the changes. Double click to install.

Outlook 2016/2019/365 Outlook 2013 Outlook 2010
Outlook 2007

Remember to restart Outlook or reboot if the registry key does not appear to work!

To reverse the changes, delete the key from the registry or edit the .reg file in Notepad and change the following line, then save and run it.

"DisableTNEF"=dword:00000001 to "DisableTNEF"=-

In addition to setting the registry key (which didn't appear to have an effect), tristof cleared the cache file:

(I went) one step further by clearing the whole auto-complete cache of Outlook. Outlook > File -> Options -> Mail -> Send Messages section. Click the "Empty Auto-Complete Lists" which will clear the entire cache and it finally worked.

Tools to Recover Attachments from Winmail.dat