Winmail.dat files are proprietary to Microsoft applications and are not able to be viewed within the web interface. You can, however, view them when reading your email in Microsoft Outlook or by using a 3rd party winmail.dat viewer.
To prevent sending winmail.dat files, the sender should set the default email format in Outlook to Plain Text and NOT use Microsoft word as the default editor for email messages.
To have the sender configure this setting:
1. Open Microsoft Outlook / Outlook Express
2. Click Tools
3. Click Options
4. Click the Mail Format tab
5. Set Compose in this message format to Plain Text
6. Un-check the check box to use Microsoft Office Word to edit email messages