How to handle a winmail.dat attachment
Tip Category:
Have you ever received an email message with an attachment named winmail.dat? If so, you've probably wondered what it was and what you should do with it ... Here's how you can handle it - you can convert it using a free utility program, but you can also tell the sender of the mail message containing the winmail.dat attachment that there are measures they can take to avoid sending you problematical attachments in future. Apple has a support article on the topic (Mac OS X Mail: What is a winmail.dat attachment?), which includes the following advice:
Products Affected Mac OS X Mail, Microsoft Outlook, Microsoft Exchange Client
The attachment means the email was sent from a Microsoft email application (such as Outlook and the Microsoft Exchange Client) and includes rich text information--how fonts, colors and specialized attributes such as underlining or boldface should appear in the email. (The attachment doesn't appear in Outlook but does appear in Mac OS X Mail; it may appear in other email applications as a MIME section named "application/ms-tnef").
Avoiding these attachments
To avoid seeing these attachments in the future, you ask the sender to deselect the email's "Send to this recipient in Microsoft rich text format" checkbox or preference setting in mail client before they send the message.
Additional Information
For more information please see this Microsoft support document.
Another source of helpful info ("winmail.dat" attachments are driving me nuts!) comes to the same conclusion:
The real fix is for the sender to make a few tweaks to their Microsoft Outlook to ensure that their copy of Microsoft Exchange doesn't think it's sending to another Outlook user. It's easy: they just have to indicate in their address book entry for you that you cannot receive Rich Text Format (RTF) rather than the current setting which assumes that you can and therefore must be running Outlook!
I use a small free utility to translate winmail.dat files. Below is a link to this free utility for Mac users - TNEF's Enough.