winmail.dat files are Microsoft's way of encoding 'rich text format' emails from Microsoft Outlook. They contain formatting information when messages use Microsoft's proprietary TNEF technology, but they cannot be read by many other email clients.
The easiest way to handle them is to get your correspondent to not send you 'rich text' format emails. Here's are some resources that explain how to do this:
The senders can also ask their IT dept to configure Microsoft Exchange not to send winmail.dat files outside their corporate firewall, which is considered best practice amongst those who run Exchange servers.
Alternatively, you can download a TNEF utility, and then use that as the "helper application" when you receive a winmail.dat file. Some users have reported success in using the following utilities:
Please note that these utilities are provided by 3rd party developers, and are not developed or tested by Postbox.