When you failover to a cluster member server, we periodically check to see if the primary home mail server is back up and running This is called Failback). If it is, we then failback to the primary server. If you are seeing a lot of those messages in the log it is indicating that we are failing over for whatever reason to the cluster mate and then failing back to the primary server over and over.
The question is, 'What is causing the failover?'
You can try the following debug:
debug_threadid=1
console_log_enabled=1
LogStatusBar=1
CLIENT_CLOCK=30
client_clock_server_format=1
CONSOLE_LOG_MAX_KBYTES=256000