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/vnd.ms-tnef .
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). 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) prefer MIME, but may still use TNEF under some circumstances (for example, if an Outlook feature requires it).
Exchange Server 
|This section does not cite any references or sources. (April 2010)|
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 (Request for Comments) compliant because these messages have the following characteristics:
- They may include non-ASCII characters outside the 0–127 US-ASCII range.
- The lines in these messages are often too long for transport via SMTP.
- They do not follow the CRLF.CRLF line termination semantics as specified in RFC821.
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.
- TNEF Parse - Claws Mail plugin
- LookOut — Mozilla Thunderbird extension
- see also:
- Tnef — Evolution plug-in
- Wireshark — Version 1.2 and later can open and dissect winmail.dat files, although it is not useful as a mail reader
Unix-like/POSIX command-line 
- yTNEF — GPL TNEF extractor from the POSIX command-line, designed specifically for reading winmail.dat
- TNEF — GPL TNEF extractor from the POSIX command-line
- KTNEF — GPL TNEF extractor for KDE
- Klammer - Shareware for Mac OS X, available on App store; also supports MSG files
- TNEF's Enough — Freeware decoder for Mac OS 9/X
- tnefDD — Free GPL'd decoder with drag-and-drop functionality for Mac OS X
- TNEF MacPorts version of the TNEF command-line decoder Mac OS X
- Letter Opener (previously known as OMiC) — Shareware, plugin for Apple Mail
iPhone and iPad 
- WinMail.dat Viewer - Browse Outlook winmail.dat files — Paid Universal app for the iPhone, iPod Touch and iPad to open winmail.dat attachments with option to view, save and share extracted files
- Winmail File Viewer — Paid Universal app for the iPhone, iPod Touch and iPad to open winmail.dat attachments
- Winmail File Viewer+ — Paid Universal app for the iPhone, iPod Touch and iPad to open winmail.dat attachments with option to save and share extracted files
- Klammer - open EML, MSG and Winmail.dat files - Paid Universal app for the iPhone, iPod Touch and iPad with in-app purchase to provide functionality
- WinDat Opener - Paid app for the iPhone, iPod Touch and iPad
- Letter Opener — Free Universal app for the iPhone, iPod Touch and iPad with in-app purchase to provide functionality
Microsoft Windows 
- Winmail.dat Reader Freeware easy-to-use decoder
- Winmail Opener Freeware decoder
- tnef2win Freeware decoder
- Free File Viewer Freeware decoder
- fentun Freeware decoder
- Winmail.dat Reader Free Android app also available
- Winmail.dat Reader Online - The online version of the Winmail.dat Reader (free)
- tud.at service — Web service and GPL licensed PHP script for reading attachments in winmail.dat files
- www.winmaildat.com - Another online winmail.dat extractor which allows to upload up to 5MB
- the-computer-site.com - converts your winmail.dat file to a zip file.
Software libraries 
- JTNEF — GPL Java TNEF package
- POI-HMEF - Java library of the Apache POI project
- MVCOM - Commercial COM Component to parse TNEF
- Convert::TNEF -- TNEF library written in Perl
- tnefparse library — TNEF access library written in Python, licensed under LGPL
- "Some Microsoft Media Types for registration". IANA. Retrieved 2010-10-25.
- "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.
- "Description of Transport Neutral Encapsulation Format (TNEF) in Outlook 2000". Microsoft. Retrieved 2009-09-11.
- "How e-mail message formats affect Internet e-mails in Outlook". Microsoft. 2005-03-30. Retrieved 2006-10-13.
- Microsoft Support:
- "How e-mail message formats affect Internet e-mails in Outlook" - How to disable TNEF in Outlook 2002 and 2003
- "Description of Transport Neutral Encapsulation Format (TNEF) in Outlook 2000" - How to disable TNEF in Outlook 2000
- "How to configure Internet e-mail message formats at the user and the domain levels in Exchange Server 2003" - Includes instructions on disabling TNEF