Prevent Winmail.dat Attachments from Being Sent in Outlook

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.

Disabe Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default

The standard settings for outgoing mail formats in Outlook can be overridden per email address. So, on a per case basis — when somebody complains about an inexplicable “Winmail.dat” attachment after you have made all the right settings changes —, you may have to reset the format for individual addresses:

  • Search for the desired contact in your Outlook Contacts.
  • Double-click the contact’s email address.
    • Alternatively, click on the desired email address with the right mouse button and select Outlook Properties… from the menu.
  • Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format:.
  • Click OK.

Thanks to About:Email for this information