What is a winmail.dat attachment?

You may wonder what winmail.dat attachments are – attachments that Apple Mail for example displays with a “question mark-over-text” icon.

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 Apple Mail; it may appear in other email applications as a MIME section named “application/ms-tnef”).

To avoid seeing these attachments in the future, you should ask the sender to deselect the email’s “Send to this recipient in Microsoft rich text format” checkbox or preference setting in her mail client before she sends the message.