Snapshots of the database, located in the mboxlist directory, are taken automatically, by default, once every 24 hours. Snapshots are copied by default into a subdirectory of the store directory. By default, there are five snapshots kept at any given time: one live database, three snapshots, and one database/removed copy. The database/removed copy is newer and is an emergency copy of the database which is put into a subdirectory removed of the mboxlist database directory.
If the recovery process decides to remove the current database because it is determined to be bad, stored will move it into the removed directory if it can. This allows the database to be analyzed if desired.
The data move will only happen once a week. If there is already a copy of the database there, stored will not replace it every time the store comes up. It will only replace it if the data in the removed directory is older than a week. This will prevent the original database which had the problem from being replaced too soon by successive startups.