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What is the winmail.dat attachment?

If a sender sends you e-mail from an e-mail program that uses Microsoft Exchange Server (such as Microsoft Outlook), an attachment named winmail.dat may be attached to the message. This file contains format information for messages and their attachments that use Microsoft's TNEF standard. This file is not recognized by other e-mail programs and ensures that all other attachments to the original message are not visible. Therefore, the attachments are not displayed in Thunderbird.

When you try to open the winmail.dat file, you are likely to be asked which program should be used to open the file. Since the file is created in Microsoft's proprietary Outlook / Exchange format, you have probably not installed a program that can decrypt and display this file. But even if you can open 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 change various settings. For information on how to do this, see this Microsoft help article.

There is also an extension for Thunderbird called LookOut. This tries to decrypt the TNEF attachments (winmail.dat) so that the attachments of the original message are correctly displayed in Thunderbird. This extension is not provided or supported by Mozilla and their compatibility with later versions of Thunderbird is not guaranteed. The best solution is to contact the sender of the message and let them know that their Outlook is set incorrectly, as described in the Microsoft help article linked above.

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