How to prevent Outlook from Formatting Email as winmail.dat

Step 1: Open up your Outlook email client
Step 2: Click on File located in the upper toolbar.
Step 3: Select the Options setting.
Step 4: From here, select the Mail category.
Step 5: Now, under Compose messages, you want to ensure the HTML or Plain Text option is selected for the Compose messages in this format.

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Step 6: Next, underMessage format, set When sending messages in Rich text format to Internet recipients to Convert to HTML format or Plain Test format.
Step 7: Click OK to finish.