LookOut is a plugin which allows will allow Thunderbird to interface with Microsoft's mail tools by decoding metadata and attachments encapsulated/embedded in a TNEF encoded attachment. The TNEF decoding engine was inspired (with permission) by the project.

From Wikipedia
Transport Neutral Encapsulation Format or TNEF is a proprietary e-mail attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most usually called winmail.dat or win.dat and has a MIME type of Application/MS-TNEF.

There are a few tnef decoders out there that do an excellent job. One could simply use one of these decoders as a helper application. This extension was created for two reasons. The first being that LookOut allows you to see what files are hidden in the anonymous encapsulation (winmail.dat). The second reason is the realization that some lack the desire to opperate more than one or two nonintegrated applications and LookOut makes the decoding part of the application. I would throw up a screenshot but that would be useless as LookOut is designed to be in the shadows doing it's thing and not be seen.

It should be noted that TNEF encapsulations will put a higher net and proc load on the IMAP server (or any other server for that matter). This is because there is no way for the client to ask for a sub-part of the TNEF file (whereas it can ask for a MIME part). If the client is to extract any sub-parts of the TNEF part it will need them all.

I can also make metadata from within the TNEF file available in the header pane but I have not coded that yet. For a list of this data (and for S&G) follow this link

Lookout now works with TB 2, download it here

Special thanks to A.J. Kehoe IV, Albrecht Gebhardt, Magnus Ihse Bursie, and Paul Libert for patches.


If you find utility in this software please consider making a donation to the ALS Hope Foundation. Also, please send an email so I can keep track. Thank you.


When LookOut is working, you will see the names of the TNEF encapsulated file attachments next to the winmail.dat file. The user interaction is transparent i.e. There are no special dialogs or menu items, etc.

Known issues

  • FIXED this only does something (visible) for file attachment bearing winmail.dat's
  • FIXED permission problem on OSX
  • save-as just brings up the open dialog
  • FIXED the RTF body replacement decoder is disabled at the moment for odd behavior
  • it is possible that if there is high server lag that you will have to click-off and click-on the message for it to work.

The lookout project can be contacted through the mailing list or the member list.
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