Transport Neutral Encapsulation Format or TNEF is a proprietary E-mail attachment format used by Microsoft Outlook and Microsoft Exchange Server.

Some TNEF files contain information used by only 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.

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, some versions of Outlook prefer MIME, but may still use TNEF under some circumstances (for example, if an Outlook feature requires it).

See http://en.wikipedia.org/wiki/Transport_Neutral_Encapsulation_Format

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