It's Outlook's fault, in a way. Or the recipient's email client's. If Outlook sends a message using the RTF format (which is not very common outside Outlook) for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file.
Receiving email clients that do not understand the code therein display it as a stale attachment. To make matters worse, Outlook may also pack other, regular file attachments in the winmail.dat file.
To prevent Outlook from attaching winmail.dat when you send an email
- Select Tools | Options... from the menu.
- Go to the Mail Format tab.
- Under Compose in this message format:, make sure either HTML or Plain Text is selected.
- Click Internet Format.
- Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format:
- Click OK.
- Click OK again.