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.

Or if you receive mail from people who use Outlook, the messages they send you might contain a "winmail.dat" attachment instead of the file 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 read using other copies of Outlook, and not by non-Microsoft mail programs or our Webmail system.

To prevent this problem, you (or the person writing to you) should make Outlook send mail using a standard format. This Microsoft page explains how to do so.