When Outlook sends messages using the RTF format for text enhancements, it includes the formatting commands in a winmail.dat file alongside the email.
Receiving email clients that have not programmed this method into their application display this as an attachment to recipient. Outlook may also pack other file attachments into the winmail.dat file including PDF Files and Word Documents.
The following instructions detail how to prevent this from occurring.
1. Open Outlook and select File.
2. Select Options.
3. Select Mail.
4. In the Compose messages section, open the Compose messages in this format dropdown menu and select HTML.
5. Scrolling down to the bottom of the Mail page under the Message format section, open the When sending messages in Rich Text format to Internet recipients dropdown menu and select Convert to HTML format.
6. Select OK and restart Outlook.
Outlook will no longer send attachments as winmail.dat files.