It's common to find attachments in Outlook email messages which are named 'winmail.dat'. Rich text and attachments can be extracted from this files with the help of EOL Soft's Winmail Opener.
In this example from the EDRM Enron Email Data Set we have an email with a small 4 KB .dat file attached. The inclination might be to ignore it, as non-convertible formatting detail from another message.
However if we simply save this attachment to a local drive and then browse and open it with Winmail Opener . . .
. . . . a quite substantive message is revealed. In this example, if we read down further in the thread, we see that the Enron employees recognized the problem with the winmail.dat attachment, and restored to faxing the message instead.
This problem arises when a winmail.dat file is used by Outlook to preserve rich text formatting, but changes made during transport prevent the receiving client from recognizing it.