Microsoft Outlook, at times, uses a TNEF format that sends email as a winmail.dat file attachment. This format can be opened by email clients such as Microsoft Outlook, Mozilla Thunderbird and Claws mail on Windows and Linux.
However, if you use the Evolution email client on Linux, you can still open the attachment indirectly, but you need to perform the following steps (at least till Evolution comes up with a solution):
Login as root.
# yum install ytnef
This installs the ytnef program. Use the following command to get to the contents of the winmail.dat file
$ ytnef -F -f . winmail.dat
The contents of the email will be extracted to your local folder. Alternatively, if this is not easy for you to do, just open that email using your email provider’s web front-end.