Kurt Keller's Blog

Some windows users have their email client configured to send all attachments (and alternate versions of the email itself) in a Microsoft proprietary format called TNEF. This causes all attachments to be combined in a single attachment with the name winmail.dat. Unfortunately only Microsoft Exchange/Outlook users can use such attachments. So what to do?

1. Ask the users sending such attachments to configure their environment properly. Two pages which explain how to do that:
   • https://www.lifewire.com/prevent-sending-winmail-dat-attachments-1173717
   • https://www.slipstick.com/problems/outlook-is-sending-winmail-dat-attachments/
2. For thunderbird users, there is the plugin called LookOut which hasn’t been updated for a couple of years, but (as of end of 2016) still works well with recent thunderbird versions.
3. And finally, for the command line and console based readers, such as mutt, there is an utility called tnef. The tnef manpage has a nice explanation how to configure this in mailcap and thus make it usable with most mail readers, including mutt.
4. Finally, for squirrelmail there is the TNEF Decoder plugin.

So, while the best solution is to simply not use proprietary attachments such as TNEF, there are some ways out there to decode them if you really, really, really need to do so.