AFAICT the -u update option currently relies on comparing the date stored in the listinfo.db file with the modification times of the mbox files. It does not check if the .msgsum file even exists. Also the date stored in the listinfo.db file is the current time. The source file may have been copied from another system, so may meanwhile have been updated further. If the source is not further updated, next time the copy may be older than the date in listinfo.db. Ideally the 'last updated date' in the cache files and listinfo.db should be the same as the most recent source from which the file is created/updated.