Fedora Cores 5 through 8 ship with GCC 4.1. They provide the libstdc++.so.6 library.
Due to the fact that Bitdefender Security for Mail Servers was compiled using an older version of the C compiler, the installation script looks for another version of that library, namely libstdc++.so.5. In order to satisfy this dependency, you must install the following package: compat-libstdc++-33.
Using yum, issue the followind command (logged in as root):
# yum -y install compat-libstdc++-33
Afterwards, you can retry installing the product.