Originally published and re-distributed upon request at: http://blogs.msdn.com/scottos/archive/2008/12/05/office-communicator-clients-cannot-communicate-with-contacts-homed-on-yahoo-messenger-network.aspx
Yahoo! will be undergoing an emergency maintenance from 4:30pm to 4:30am PST. During this period, users will see intermittent presence issues. All other functionality will not be impacted.
After the maintenance window expires, if you find you are still having issues, please first log out & then back into your Communicator client. If the issue still reproduces/occurs for you, please restart Edge Front-End services first. Allow me to apologize for this up front; I understand this will require an “emergency service restart change request” for some of you. If all this fails to resolve the PIC issue between your LCS/OCS deployment and Yahoo!, please engage Microsoft Customer Support Services. Premier customers: please leverage your Technical Account Manager to initiate the case creation process. Please be prepared to supply Edge Server logs, remote access via our EasyAssist applications from MSFT, and we will do our best to investigate and resolve this in a timely manner.