Apple mail boxes (.mbox file) are packages. Select the mbox file on the finder, right click or control-click to display the contextual menu and select the 'Show package contents' menu. From the new Finder window select the 'mbox' file and drop it onto eMail Extractor.
Due to the fact several files in mbox packages are binaries (content_index, table_of_contents...) eMail Extractor can have problems processing the whole package at once. It is better to drop the 'mbox' file alone.
