Winmail.dat attachments

Some Microsoft email products put attachments into a proprietary format called Transport Neutral Encapsulation Format (TNEF), which is not immediately readable by non-Microsoft email clients.

When this occurs the receiving email client will show a useless attachment called winmail.dat.

Turning off TNEF at the source

The ideal solution, providing best compatibility, is for the sender to turn TNEF off so that their emails are not using proprietary formats that are not readable by non-Microsoft email clients.

When Outlook is configured to send e-mail in "Outlook Rich Text Format", it may use TNEF. When it sends in "HTML" or "Plain Text", it uses standard, compatible formats. There are two options for disbling TNEF:

In at least Outlook 2002 (a.k.a. Outlook XP) and Outlook 2003, if it's being used at a business, the following is recommended to ensure compatibility with corporate mail systems:

On the "Tools" menu, click "Options", then click the "Mail Format" tab, and then the "Internet Format" button.
Set "When sending Outlook Rich Text messages to Internet ..." to either "Convert to HTML format" or "Convert to Plain Text format".

In all versions of Outlook, you can disable TNEF completely:

On the "Tools" menu, click "Options", and then click the "Mail Format" tab.
In the "Send in this message format" list, click "Plain Text" or "HTML", and then click "OK".

Source: kb.mozillazine.org/Winmail.dat_attachments

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Reading winmail.dat attachments

If you are receiving winmail.dat attachments, we recommend that you notify the sender of the problem you are having. However, in some circumstances, you may want to just go ahead and decode the attachment into its composite parts (i.e. one or more useable attachments).

In the Thunderbird email client, we recommend the LookOut extension which will run conveniently from within your Thunderbird email client.

There are also other applications that may be useful, including TNEF's Enough (freeware for Mac) and TNEF, yTNEF, or KTNEF (GPL for Linux).