Transport Neutral Encapsulation Format

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File Format
Name Transport Neutral Encapsulation Format
Extension(s) .dat, .tnef
MIME Type(s) application/
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.



Such mail files start with signature bytes 0x78 0x9f 0x3e 0x22.


  • 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
  • XAD framework

Sample files

Links and references

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