Be sure the program is being "Run as Administrator" or it will fail to decode properly.
Some computers may give security warnings when trying to run the program, these are usually due to default security settings and can safely be ignored/agreed to.
In order to set the program to run as admin:
Right click on the EmailDecypt.exe file
Select "Properties" from the list
Navigate to the "Compatibility" tab at the top
Check the "Run this program as an administrator" box near the bottom
The program should now run correctly but may incite a "User Account Control" prompt which you can simply agree to and the program will run.
To redownload the program click here.