By using the intrusion protection system you have the possibility to block all kind of traffic. So it is possible to block malicious code or 'attacks'. But it is also possible to block P2P clients, FTP, special kinds of ping, MSN, ...
I assume you enabled a IPS rule which blocks MSN traffic. Have a look into the IPS Live Log to see which packets are dropped by which IPS rule. Just disable the rules which are responsible for your droped MSN connection.
