Most often resolving problems with opening TNEF files is very simple. Just install an appropriate program that supports such files. All of the listed programs support TNEF files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of TNEF files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate TNEF files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Apple Mail applications is not on the list, select "Browse" option in order to manually locate the directory where Apple Mail has been installed.
Sometimes problems with opening TNEF files may not lie with the application itself, but can arise due to other causes, such as: