To connect from behind a firewall
Sign in to Windows Messenger.
You should automatically connect to instant messaging services―even from behind a firewall.
Windows Messenger will first attempt to connect directly so that you can use its full functionality. If Windows Messenger cannot connect directly, it will use the same HTTP connection that your default browser uses to connect to the Internet.
Important: If Windows Messenger uses your browser connection, you'll be able to send instant messages, but some features of Windows Messenger―such as placing calls, sending files, or connecting to other messaging services―may not be available.
Notes
To find out what kind of connection you have, go to the Connection tab of the Options dialog box (Tools, Options). In the bottom half, you'll see the type of connection you have―for example, "You are currently connected to Windows Messenger using your default browser's settings."
To get full functionality and avoid using your default browser connection, ask your system administrator to refer to the Guide for Network Administrators and the topic Configure your network for voice and video communications, application sharing, Whiteboard, and Remote Assistance.