Just as side commentary...this one has been a long time coming.

Many people often overlook the need to do the following, even though it is documented:
1) Configure an A record in DNS for the LCS 2005 Enterprise Pool(s)
2) Use the Pool name with LCS 2005 Enterprise Edition, even in a single-server deployment.
(Note: For LCS 2005 Standard Edition and LCS 2003 the pool name and the server name are synonymous. If you use the server name, in Windows Messenger, you are also using the pool name).
3) Configure MTLS/TLS when more than 1 server, even two servers in the same pool, are implemented in their LCS environment.

The article:
You receive a "The following message could not be delivered to all recipients" error message when you send an instant message to a user in Live Communications Server 2005 Enterprise Edition
http://support.microsoft.com/kb/898150

Here is another good article related to LCS connectivity issues:
"An HTTPS or TCP connection could not be made