Outlook: attachments are getting converted to winmail.dat

The Microsoft Outlook e-mail program sometimes sends e-mails in the Transport Neutral Encapsulation Format (TNEF). Most other e-mail programs, including Thunderbird, do not understand TNEF.

If your e-mail program doesn’t understand TNEF, instead of seeing the e-mail and/or attachment, you may only see an attachment named “winmail.dat” or “Part 1.2” that you cannot open. Also, sometimes you may receive a TNEF attachment with a generic name such as ATT00008.dat or ATT00005.eml instead.

Microsoft Easy Fix 

Outlook 2010

To prohibit Outlook 2010 from sending the Winmail.dat file, follow these steps:

1. Add the DisableTNEF registry entry. To do this, follow these steps:
   1. Exit Outlook 2010.
   2. Start Registry Editor.
      • In Windows 7, click Start
        

        , type regedit in the Search programs and files box, and then press Enter.

      • In Windows Vista, click Start
        

        , type regedit in the Start Search box, and then press Enter.

        

        If you are prompted for an administrator password or for confirmation, type the password, or provide confirmation.

      • In Windows XP, click Start, click Run, type regedit, and then click OK.
   3. Locate and then select the following registry subkey:
      HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
   4. On the Edit menu, point to New, and then click DWORD Value.
   5. Type DisableTNEF, and then press Enter.
   6. Right-click DisableTNEF, and then click Modify.
   7. In the Value data box, type 1, and then click OK.
   8. Exit Registry Editor.
   9. Restart the computer.

Outlook 2007

To prohibit Outlook 2007 from sending the Winmail.dat file, follow these steps:

1. Install hotfix 957692  .For more information, click the following article number to view the article in the Microsoft Knowledge Base:
   957692  Description of the Outlook 2007 hotfix package (Outlook.msp): October 28, 2008
2. Add the DisableTNEF registry entry. To do this, follow these steps:
   1. Exit Outlook 2007.
   2. Start Registry Editor.
      • In Windows Vista, click Start
        

        , type regedit in the Start Search box, and then press Enter.

        

        If you are prompted for an administrator password or for confirmation, type the password, or provide confirmation.

      • In Windows XP, click Start, click Run, type regedit, and then click OK.
   3. Locate and then select the following registry subkeys:
      HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
   4. On the Edit menu, point to New, and then click DWORD Value.
   5. Type DisableTNEF, and then press Enter.
   6. Right-click DisableTNEF, and then click Modify.
   7. In the Value data box, type 1, and then click OK.
   8. Exit Registry Editor.
   9. Restart the computer.

• Select Tools | Options… from the menu.  <in outlook 2010 Outlook 2010: File-> section Options-> )
• Go to the Mail Format tab.
• Under Compose in this message format:, make sure either HTML or Plain Text is selected.
• Click Internet Format.
• Make sure either Convert to Plain Text format or Convert to HTML format is selected underWhen sending Outlook Rich Text messages to Internet recipients, use this format:
• Click OK.
• Click OK again.

Disabe Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default

The standard settings for outgoing mail formats in Outlook can be overridden per email address. So, on a per case basis — when somebody complains about an inexplicable “Winmail.dat” attachment after you have made all the right settings changes —, you may have to reset the format for individual addresses

• Search for the desired contact in your Outlook Contacts.
• Double-click the contact’s email address.
  • Alternatively, click on the desired email address with the right mouse button and select Outlook Properties… from the menu.
• Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format:.
• Click OK.

More info