What does the error Spool file is locked mean?

This is not an error. All it means is that when an Exim delivery process (probably started by a queue runner process) looked at a message in order to start delivering it, it found that another Exim process was already busy delivering it. On a busy system this is quite a common occurrence. If you set -skip_delivery in the log_selector option, these messages are omitted from the log. The only time when this message might indicate a problem is if it is repeated for the same message for a very long time. That would suggest that the process that is delivering the message has somehow got stuck.

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