Whats up with MSN Messenger and NAT?
File Transfers You can receive files but not send them. Mapping the appropriate port (6891) will not improve the situation. The reason is that IP addresses are passed as data in the messages exchanged in setting up the direct, user-to-user file transfer connection. If you offer a file for download, the private address of your computer is sent to the receiver in the connection setup messages and the subsequent (inbound) connection attempt therefore fails. When receiving a file, the file transfer connection is outbound and hence, NAT poses no problems. Other features The other features (e.g. audio and video) use SIP and will not work through standard NAT, although outbound connections may still succeed (you may be able to hear the other end but not vice versa for example). See here for more information on this. The fix Messenger is UPnP aware, so if you have an UPnP aware router, Messenger can retrieve its public IP address to pass in connection setups and can also dynamically map ports
