MSN, also called as live messenger is widely used. Windows Live Messenger gives you brilliant ways to connect and share your photos (and other stuff). Contact lists, emoticons, instant access to your friends.
However, sending and receiving files using MSN will face some security risk. External users can send files that might contain viruses or malicious code to users on the internal network. In addition, a liability risk arises if employees use the file transfer feature to share copyrighted music, movie or software files in violation of the law.
How to block msn file transfer?
MSN transfers files using dynamic ports which are negotiated. So it is impossilbe to block msn file transfer ports.
A more detailed example can be found here: