Opening winmail.dat files from Outlook attachments

Occasionally I receive emails from people which have a single attachment named winmail.dat. After a bit of searching online, it turns out that Microsoft Outlook, depending on how it is configured, uses the MIME type application/ms-tnef for attachments, which other mail clients (e.g. Thunderbird) can’t read.

Unfortunately I couldn’t find a simple way to patch other mail clients to read these files, and the extension that was available for Thunderbird has not been updated for over 7 years and so is unlikely to work. However, someone has helpfully created a program for Linux called TNEF, with the source code available on GitHub.

Installation is relatively simple, I used the following steps:

git clone
cd tnef
./configure --prefix=/home/paul/bin/tnef
make check
make install

To extract the files contained within winmail.dat, save the attachment locally (e.g. to /tmp) and then run:

tnef -f /tmp/winmail.dat -C /tmp/

It might be possible to automate the above steps so that Thunderbird calls tnef with the appropriate options, but I haven’t had time to investigate that. Please let me know in the comments if you’re aware of an alternative option which would integrate with the latest versions of Thundebird (60.x).


  1. Paul

    That does appear to do the trick – thanks!

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