This option enables extra consistency checking that is only necessary if you have a mail program that will modify the INBOX file without changing the time or the size of the file.
Regular consistency checking depends on file-time and file-size to determine if the index file needs to be rebuilt.
The consistency checks option enables additional consistency checking where every message is checked to see if it begins at the write offset in the file and the status flags are also checked to see if they match the cached copy in the index file.
To enable checking: