An mbox mail file is contains plain emails concatenated in one big file. Since each mail starts with "From ", and ends with a newline, they can be separated from each other.
With the help of this class, you also can insert(), remove() and update() messages in the mbox file. When calling one of this methods, the class checks if the file has been modified since the index was created - changing the file with the wrong positions in the index would very likely corrupt it. This check is not done when retrieving single messages via get(), as this would slow down the process if you retrieve thousands of mails. You can, however, call hasBeenModified() before using get() to check for modification yourself. If the method returns true, you should close() and re-open() the file.
If something strange happens and you don't know why, activate debugging with setDebug(true). You also can modify the temporary directory in which changed mboxes are stored when adding/removing/modifying by using setTmpDir('/path/')