Overview

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.

Fix

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:

• Change the message format for all messages sent.
• Change the message format for all messages sent to a specified recipient.
• Change the message format for one message.