Hi, danny. I'm Robinson, and I’m happy to help you today.
Are you using Thunderbird? If yes, this is a normal problem, this happens when the sender using Outlook attaches an entire email instead of attaching only the file. For Thunderbird, there is an add-on called LookOut that is able to decode the winmail.dat and display the original attachments in Thunderbird's message pane.
You can see more information and see how to install the add-on at this link.
https://support.mozilla.org/en-US/kb/what-winma...
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.