Microsoft Outlook and its Exchange Client uses a method called Transport Neutral Encapsulation Format (TNEF) to transport data across a Network.

On occasion, there are settings within Outlook that can hinder this process and make the attachment of an email unreadable. In other words a 'Winmail.dat' file.
The Winmail.dat file includes the original message that was sent, along with its binary encapsulation.

To prevent receiving this in future, it is important to change some of the settings within the Outlook Application.

To do this:
- Open Outlook
- Go to: File
- Then Options. This should open a new box labelled 'Outlook Options'
- From here, select Mail
- Under the heading 'Compose Messages', change the format of 'Compose Messages in this format' to either 'HTML' or 'Plain Text'
- Scroll down until you can see the heading 'Message Format'
- Change the setting for 'When sending messages in Rich Text Format to Internet Recipients'. This should read either 'Convert to HTML Format' or 'Convert to Plain Text Format'

If these settings have been changed and the file being received is still Winmail.dat. Remove the recipients contact details from the address book, then attempt to send the email with the attachment again.

This should prevent Transport Neutral Encapsulation Format and Microsoft's Rich Text Format from being an issue again.