Why do I receive error notifications that a mailbox is locked?
Oracle Service Cloud, all versions prior to November 2016
These notifications are expected from techmail under certain conditions. This message indicates that the locking mechanism is working as intended. The root cause of this message is due to multiple message processors competing for the same resources. Because one process is able to obtain the lock, the other cannot and thus returns this message. The impact to a site is very minimal considering a process is already running. Any message not processed immediately is left in the mailbox and processed in subsequent runs.
Because this is a legitimate message, notifications were moved from the error logs to the info logs starting in the November 2016 release. Again, this message should not cause alarm and can be ignored in most cases.