Before you installed Windows Messenger 5.0, you had no problems transferring files to another Windows Messenger user with a previous version of Windows Messenger.
This problem will occur if your computer is multihomed, or your network adapter had multiple bindings. In this environment, Windows Messenger 5.0 sends an incorrect IP address to the recipient Windows Messenger client.
NOTE: A multihomed computer has more than one network adapter installed.
To resolve this problem, install the latest version of Windows Messenger 5.0.