How to access MacBook mail preview files after the email is closed?
Open mail and then export all the messages to a file in the .mbox standard format. 


https://support.apple.com/kb/PH19174


There is wide support for parsing that file format or importing it into other clients if you wish. Apple's format changes for time to time and is optimized for tasks other than easy parsing like spotlight search, minimizing file system changes for time machine, etc...