Microsoft Outlook offers several different formatting options when composing messages, one option is RTF (Rich Text Format) which includes text formatting enhancements.

RTF is a Microsoft format only supported by Microsoft Outlook. If you send a RTF formatted email to someone using an email client other than Outlook they may receive a Winmail.dat file attachment with the message because it contains instructions on how other email clients should format the text.

This only happens when composing in rich text format because plain text format does not have the ability to add text enhancements, and HTML format includes all the enhancements in the HTML code itself, so there is no need for a winmail.dat file.

You can prevent Microsoft Outlook from sending winmail.dat attachments by changing the format Outlook is using.

Click the file menu and then select options.

Select the mail category from the menu. In the compose messages section, use the “Compose messages in this format” drop-down and choose either HTML or plain text.

Scroll down and in the message format section, use the “When sending messages in rich text format to internet recipients” drop-down and choose either convert to HTML format or convert to plain text format.

Click OK when finished.