For all the features of MSN to work there are multiple ports you need to open;
I can't be sure that my list is up to date...but I believe it is as follows;
TCP 6891 - 6900
TCP 1863
UDP 1863
UDP 5190
UDP 6901
TCP 6901
If I've missed any could anyone point it out...as for connecting to the MSN Server (64.4.13.58) via port 80 I don't know if it's possible, the default port, to my knowledge, is 1863...Plus! couldn't do this, it hooks in and doesn't alter the connection side of MSN Messenger...There's a program at
http://www.hopster.com/ that can bypass ports that IM programs use (compatible with MSN Messenger) that should allow you to get through your firewall, however before using it open the ports I suggested...