Have you received an email with a winmail.dat attachment? If so, you may have been sent an email from an Outlook client that thinks it’s talking to another Outlook client. When Outlook sends mail in “Outlook Rich Text Format”, which is used primarily for Exchange-only communication, attachments and potentially other components of the email can be attached in Transport Neutral Encapsulation Format. This shows up to most non-Outlook mail clients as an attachment named winmail.dat.
To stop this from happening, the Outlook client needs to be configured to not send emails in “Outlook Rich Text Format” to clients other than Outlook and Exchange clients as in this Office support article. There are three ways to make this change: