Many organizations face a productivity problem with employees who abuse chat programs like MSN Messenger. Some IT departments are instructed to block this kind of traffic for users who either abuse or simply don't require the software. You can block applications like MSN Messenger in your proxy server, but some clients may still have access to the applications, because there are many versions of MSN Messenger in use, making blocking the application with a proxy server difficult. Instead, block MSN Messenger traffic more easily with iptables.