Sustainability of Digital Formats: Planning for Library of Congress Collections

Transport Neutral Encapsulation Format (TNEF) is a proprietary email attachment format utilized by Microsoft Outlook and the Microsoft Exchange Server. It employs the TNEF Data Algorithm to encode rich properties in email messages, enabling transport as a stream, a file attachment, or a MIME entity body. Programs for decoding and extracting files from TNEF-encoded attachments are widely available across various platforms. The Wikipedia TNEF Decoding article has a list, which includes the several command-line tools, the tnefparse library, Klammer for macOS, Winmail File Viewer for iPhone and iPad, Winmail.dat Reader for Microsoft Windows, TNEF Extractor for Android, and Wireshark.

This format was developed to convey rich-text email elements (bold, italic, multiple fonts, etc.) from an Exchange email to a non-Exchange email that does not support rich text. This email will contain a TNEF attachment called “Winmail.dat.”

A TNEF-encoded message includes a plain text version and a binary attachment (typically Winmail.dat). The binary attachment "packages" various message elements, such as formatted text, Object Linking and Embedding (OLE) objects, special Outlook features, or regular file attachments.

TNEF initiates with a self-identifying 32-bit signature. Subsequently, a 16-bit unsigned integer serves as a key to cross-reference attachments to their locations in the tagged message text. The rest of the format consists of TNEF attributes. Each attribute includes the level at which it applies (message or attachment), ID, length of the contained data, the data itself, and a 16-bit checksum of the bytes comprising the data. Numeric data types larger than one byte follow the little-endian format (see: Specification section 2.1: Common Algorithm Details).

The Winmail.dat file encompasses embedded details, including the path of the personal folders file (see PST ANSI or PST Unicode) and the log on name. While this information remains hidden from recipients during regular viewing, it becomes accessible if the recipient opts to open Winmail.dat for editing using a binary or text editor. Although log on name and personal file paths may be revealed, password information is not included.

TNEF’s file extension can be .tnef, but it is more likely to be .dat. In most cases, the binary attachment is named Winmail.dat.

When an email client that does not understand TNEF receives a message containing TNEF information there are two common results:

• 1. The client may receive a plain text version with an attachment named Winmail.dat, which, when opened, lacks useful information due to its TNEF format.
• 2. the client may receive a plain text version with an attachment bearing a generic name (e.g., ATT00008.dat), indicating the client's inability to recognize either the TNEF portion or the Winmail.dat file name. In this scenario, a generic file name is generated to preserve the TNEF information.

In addition to the receiving client, it is common for an email server to strip out TNEF information from messages as it delivers them. If a server option to remove TNEF is turned on, clients will always receive a plain text version of the message. Exchange Server is an example of an e-mail server program that has the option to remove TNEF from messages.

[MS-OXTNEF]: Transport Neutral Encapsulation Format (TNEF) Data Algorithm.

Also available as a PDF: [MS-OXTNEF]: Transport Neutral Encapsulation Format (TNEF) Data Algorithm.

Used in Microsoft products. According to Exchange Server: TNEF conversion options, "all versions of Outlook fully support TNEF. Outlook on the web (formerly known as Outlook Web App) translates TNEF into MAPI and displays the formatted messages. Other email clients that don't support TNEF typically display TNEF formatted messages as plain text messages with Winmail.dat or Win.dat attachments.

Able to be read by non-Microsoft tools.

From the specification:

"This documentation is covered by Microsoft copyrights. Regardless of any other terms that are contained in the terms of use for the Microsoft website that hosts this documentation, you can make copies of it in order to develop implementations of the technologies that are described in this documentation and can distribute portions of it in your implementations that use these technologies or in your documentation as necessary to properly document the implementation. You can also distribute in your implementation, with or without modification, any schemas, IDLs, or code samples that are included in the documentation. This permission also applies to any documents that are referenced in the Open Specifications documentation.

Patents. Microsoft has patents that might cover your implementations of the technologies described in the Open Specifications documentation. Neither this notice nor Microsoft's delivery of this documentation grants any licenses under those patents or any other Microsoft patents".

TNEF has security considerations noted in the IANA registry application:

"This information is interpreted by a MAPI [Messaging Application Programming Interface] aware application on receipt and is used to re-construct [sic] the original MAPI properties on the message. One security consideration that is important to note is that some messaging implementations can include properties such as a request for a Read Reciept [sic] and Delivery Reciepts [sic]. Recipient’s mail systems often honour [sic] these requests without informing the user that a reciept [sic] is being sent back for a message".

TNEF version 0.1 was released on April 4, 2008 with the first major version (1.0) released on June 27, 2008. As of January 2024, there are 39 versions listed in the specification. TNEF is an actively managed product with more versions anticipated throughout its lifetime. As of this writing, the most recent version is 13.0 (released August 17, 2021).