Update email format settings in Outlook (Windows)
Update your default email format settings to HTML so your outgoing messages are compatible with other email clients. Incompatible email might have a file attachment called winmail.dat added to outgoing messages. They're not harmful, but they can cause confusion as they aren't recognized by other email clients. Changing your default email format can prevent winmail.dat attachments from appearing on your outgoing emails.
- Open Outlook.
- Select File > Options.
- Select Mail.
- Under Compose messages, in the Compose messages in this format list, select HTML.
- At the bottom of the window, select OK.