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Information about TNEF/winmail.dat filtering

If you have found your way to this web page, then you will probably have encountered an email error message in connection with a winmail.dat attachment or the Transport Neutral Encapsulation Format (TNEF). We will focus on TNEF encoding. This encoding is exclusively used by Microsoft® Outlook and Microsoft® Exchange Server. It is a proprietary format and no other software uses it.

  • When does Microsoft® Outlook use TNEF encoded emails?
    Microsoft® Outlook uses TNEF when the message contains Rich Text Format (RTF), Object Linking and Embedding (OLE), a form, voting buttons, and meeting requests. It can also be used if the sender forwards other content in a message.
  • Why are TNEF encoded emails blocked?
    Since no other software except Microsoft® Outlook and Microsoft® Exchange Server use TNEF encoding, there are usually the following reasons for the block.
    • The message is not recognised as email.
    • The receiving software doesn't support TNEF.
    • A security filter in between doesn't support TNEF and can't inspect the content, thus forcing a block or a different action than accepting it.
    In either case you have to investigate the reason with the help of the receiving side. Bear in mind that only the sender can change the message encoding, not the recipient and no filter in between. Microsoft® suggests not to use TNEF in connection with client other than Microsoft® Outlook and Microsoft® Exchange Server, which is usually true for email sent to unknown clients in the Internet.
  • I still do not understand the risk of TNEF encoding. Why all the fuss?
    TNEF attachments may contain security sensitive information such as user login name and file paths, from which access controls can be inferred. There are no Internet-standard email functions that require TNEF encoding, and any file that can be sent using this method can also be sent using the Multipurpose Internet Mail Extensions (MIME) standard supported by all mail servers and nearly all mail clients. However, there are some functions of Microsoft's proprietary email infrastructure (see the documentation of Microsoft® Exchange Server for details) that require TNEF. If you really require TNEF encoded messages make sure you deploy and use Microsoft® Outlook in the way the vendor recommends it. This includes activating automatic updates and using additional protective software (such as antivirus filters on the client). If you deploy and use Microsoft® Exchange Server ensure that the same guidelines are followed.
  • How can I configure Microsoft® Outlook not to send TNEF encoded messages?
    Microsoft® Outlook offers options to control the format of the messages sent. See the following articles for a documentation how to configure this. The format settings can also be controlled per recipient in the address book.
We hope to have solved the mystery of the error messages that may have sent you here.