Prevent Winmail.dat Attachments from Being Sent in Outlook
To prevent Outlook from attaching winmail.dat when you send an email:
Select File in Outlook.
Go to the Mail.
Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
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.