To prevent Outlook from attaching winmail.dat when you send an email:

  1. Click File in Outlook.
  2. Select Options.
  3. Go to the Mail category.
  4. Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
  5. Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients: under Message format.
  6. Click OK.