Stale mailbox lock file

Hi all, I would like to understand the meaning of "Stale mailbox lock file detected" In index-storage.c (https://mis.dopa.go.th/atmail/server_source/dovecot/src/lib-storage/index/in...) there is this notice "Stale mailbox lock file detected" ...

I have a java application which connects via javamail to a dovecot server and I have some anomalies when that message comes.

Thanks, bye

Raffaele Gambelli