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How-to: Stop Outlook Sending Winmail.dat Attachments

25 November 2009 by J.M.

Sometimes when you send a message with Microsoft Outlook, the recipient only receives a “winmail.dat” attachment instead of the original ones.  You can prevent this from happening by selecting the correct message format.

TNEF encoding (Transport Neutral Encapsulation Format) was invented by Microsoft and cannot be decoded properly by all mail programs.  When that happens, the recipient’s mail program only displays a “winmail.dat”.  Here is how to change this Outlook behavior:


  1. Open Microsoft Outlook.
  2. Outlook 2007 or lower: Click Tools, Options, E-mail format.
  3. Outlook 2010: Click File, Options, E-mail.
  4. Instead of Rich text or Text only, select HTML as message format.
  5. Outlook 2007 or lower: Click Internet format.
  6. Outlook 2010: Scroll down.
  7. Select Convert to HTML format in the drop down.
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