Transport Neutral Encapsulation Format

File Format
Name	Transport Neutral Encapsulation Format
Ontology	Electronic File Formats E-Mail, newsgroups, and forums Transport Neutral Encapsulation Format
Extension(s)	.dat, .tnef
MIME Type(s)	application/vnd.ms-tnef
Wikidata ID	Q1601331

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 0x78 0x9f 0x3e 0x22.

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

Links and references

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