Transport Neutral Encapsulation Format

From Just Solve the File Format Problem
Jump to: navigation, search
File Format
Name Transport Neutral Encapsulation Format
Extension(s) .dat, .tnef
MIME Type(s) application/

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.


  • 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

Sample files

Links and references

Personal tools