Error: The following message was not delivered to [email protected]

I am now using OCS 2005 SP1 server and LCS 2007 R2 server. I have a signing in LCS server username, they shows OCS servers contacts are offline. Then, I tried to send IM, but I came up with the following error- “The following message was not delivered to [email protected].” Please, tell me how to make connection/communicate between two servers.
