Why do messages contain unreadable “winmail.dat” attachments?

If you use Microsoft Outlook to send mail, people might complain that your messages contain an unreadable "winmail.dat" attachment, with the normal attachments not visible.

Or if you receive mail from people who use Outlook, the messages they send you might contain a “winmail.dat” attachment instead of the attachment you were expecting.

This happens because the copy of Microsoft Outlook that sent the message is configured to send attachments in a non-standard format. Those attachments can only be viewed using certain mail program like Outlook, and not by most non-Microsoft mail programs or our Webmail system.

To prevent this problem, the sender should make Outlook send mail using a standard format. This Microsoft page explains how to do so (we recommend their “method 4”, changing the “Outlook Rich Text options” to “Convert to HTML format”).