What is the winmail.dat attachment?

Messages sent from incorrectly configured Microsoft Outlook/Exchange email clients may contain a file attachment called winmail.dat. This attached file contains formatting information for messages that use Microsoft's proprietary TNEF standard, plus any attachments sent with the original message. The TNEF file is not recognized by other email clients, so any attachments sent with the original message are not displayed in Thunderbird's message pane.

If you try to open winmail.dat, you will probably be prompted to specify the application that should be used to open the file, unless you have an application installed that can decode this file and display it. Even if your system is capable of displaying the file, it does not contain any useful information.

To prevent this file from being attached to messages, the sender of the message (or their system administrator) can configure various options as described in this Microsoft Support article.

The best solution is to contact the message sender and inform them that their copy of Outlook is incorrectly configured as suggested in the Microsoft Support article.