The problem sending a winmal.dat attachment in the Outlook email client is broadly related to the way Outlook encodes messages. This attachment format is used by Outlook when it sends messages in Rich Text Format (RTF) instead of HTML or plain text.
To resolve this issue, please follow the steps described down below:
- Change the format of your message: Change the format of the message you send to HTML or Plain Text instead of Rich Text. You can do this by clicking "Format Text" in the message compose window and selecting "HTML" or "Plain Text".
- Change default format settings: Go to your Outlook email client settings for this change. You can do this from "File > Option > Mail > Compose messages in this format" and choose "HTML" or "Plain text".
- Check recipient settings: Make sure your recipient receives messages in HTML or plain text format. If your recipient is using an email client that does not support Rich Text format, this may result in winmail.dat being displayed.
- Use a different email client: If this problem persists, you can use another email client compatible with the message format you prefer (e.g.: Thunderbird).