For instant messaging
Set outgoing Transmission Control Protocol (TCP) connections to port 1863 on your proxy servers.
On the main window, click the Tools menu, click Options, and then click the Connection tab.
Note the information that is required on this tab.
Provide all network users with the information and instructions they need to properly configure the Connection tab of their Windows Messenger program.
Make sure the internal local area network has access to the Domain Name System (DNS) servers to resolve the names of external hosts such as messenger.microsoft.com.
For file transfer
Open as many TCP ports as you can between 6891 and 6900.
Configure the TCP ports so that sockets on a port remain open for an extended period of time.
Note
If you are using a NAT, file transfer will not work unless you have a Universal Plug and Play NAT (UPnP NAT). If you are not sure what type of NAT you have, check with your computer manufacturer or NAT vendor. Make sure that your NAT is updated to the latest version and that it is UPnP-compliant.
I hope you find this post helpful. Please let me know if it was.
Regards,
Mark