Transport Neutral Encapsulation Format
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Transport Neutral Encapsulation Format or TNEF is a proprietary e-mail attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most usually called winmail.dat or win.dat and has a MIME type of Application/MS-TNEF.
Contents
[edit] Overview
Some TNEF files only contain information used by Outlook to generate a richly formatted view of the message, such as embedded (OLE) documents or Outlook-specific features such as forms, voting buttons, and meeting requests. Other TNEF files may contain files which have been attached to an e-mail message.
Within the Outlook email client TNEF encoding cannot be explicitly enabled or disabled. Selecting RTF as the format for sending an e-mail implicitly enables TNEF encoding, using it in preference to the more common and widely compatible MIME standard. When sending plain-text or HTML format messages, Outlook uses MIME.
TNEF attachments can contain security-sensitive information such as user login name and file paths[1], from which access controls could possibly be inferred.
[edit] Decoding
Programs to decode and extract files from TNEF-encoded attachments are available on many platforms.
[edit] RFC Compliance
Native-mode Microsoft Exchange Organizations will in some circumstances send entire messages as TNEF encoded raw binary independent of what is advertised by the receiving SMTP server. As documented in Microsoft KBA #323483, this technique is not RFC compliant because these messages have the following characteristics:
