Transport Neutral Encapsulation Format


Transport Neutral Encapsulation Format or TNEF is a proprietary email attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most often named winmail.dat or win.dat, and has a MIME type of Application/MS-TNEF.
The official media type, however, is application/vnd.ms-tnef.

Overview

Some TNEF files contain information used by only Outlook to generate a richly formatted view of the message, such as embedded 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 e-mail 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, some versions of Outlook prefer MIME, but may still use TNEF under some circumstances.
TNEF attachments can contain security-sensitive information such as user login name and file paths, from which access controls could possibly be inferred.

Exchange Server

Native-mode Microsoft Exchange 2000 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:
Internal communications between Exchange Servers over SMTP encode the message in S/TNEF format. The conversion between the format needed by the end client on the Internet is performed on the last Hub Transport server before final delivery, and when the Hub Transport role of an Exchange Server is about to deliver the message to a mailbox role server, the message is converted to MAPI format for storage.
S/TNEF differs from TNEF in that it is 8-bit and does not contain a plain-text portion.

Decoding

Programs to decode and extract files from TNEF-encoded attachments are available on many platforms.

Multiplatform