Follow the steps below to prevent Outlook from attaching a Winmail.dat file when you send an email with an attachment:

1. In Outlook, click on the File tab.

2. Click on Options. 

3. Select Mail.

4. Under Compose Messages, make sure either HTML or Plain Text is selected for the "Compose messages in this format" option.

5. Under Message Format, make sure either Convert to Plain Text format or Convert to HTML format is selected under "When

sending messages in Rich Text messages to Internet recipients" option.

6. Click OK