Transport Neutral Encapsulation Format

Transport Neutral Encapsulation Format (TNEF) is a proprietary format used by Microsoft mail software to encode attachments and rich text, instead of more standards-compliant methods such as MIME.

It is usually implemented as an attachment called winmail.dat or win.dat, which Microsoft programs know what to do with, but is a useless and confusing attachment to everybody else (unless they use one of several programs and plugins that have been devised to help non-MS-ers cope with it). When saved elsewhere as archives or samples, sometimes the .tnef file extension is used.

Identification
Such mail files start with signature bytes.

Software

 * Fentun: Freeware Windows program to decode this attachment
 * TNEF: An open source TNEF decoder utility
 * Lookout: Thunderbird addon to read TNEF (based on the above TNEF decoder utility)
 * Winmail Opener
 * ytnef: TNEF command-line extractor

Sample files

 * http://libxad.cvs.sourceforge.net/viewvc/libxad/testfiles/MS-TNEF/
 * https://github.com/koodaamo/tnefparse/tree/master/tests/examples
 * https://telparia.com/fileFormatSamples/archive/tnef/

Links and references

 * Transport Neutral Encapsulation Format (Wikipedia)
 * How to turn off winmail.dat attachments
 * MS-TNEF format specifications