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 (IANA) media type, however is application/[1]


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.

Within the Outlook e-mail client, TNEF encoding cannot be explicitly enabled or disabled (except via a registry setting[2]). 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 (apparently including Outlook 2000[3]) prefer MIME, but may still use TNEF under some circumstances (for example, if an Outlook feature requires it).[3][4]

TNEF attachments can contain security-sensitive information such as user login name and file paths,[3][4] 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 (2000 and later) over SMTP encode the message in S/TNEF (Summary 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 (not 7-bit for TNEF) and does not contain a plain-text portion.


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


Unix-like/POSIX command-line


iPhone and iPad

Microsoft Windows



Software libraries


  1. "Some Microsoft Media Types for registration". IANA. Retrieved 2010-10-25.
  2. "When you use Outlook 2007 to send an e-mail message, the recipient of the message sees an attachment that is called Winmail.dat". Microsoft. 2009-07-29. Retrieved 2011-03-03.
  3. 1 2 3 "Description of Transport Neutral Encapsulation Format (TNEF) in Outlook 2000". Microsoft. Retrieved 2009-09-11.
  4. 1 2 "How e-mail message formats affect Internet e-mails in Outlook". Microsoft. 2005-03-30. Retrieved 2006-10-13.

External links

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