The description of mbox format expansion
The mbox file is mailbox text file saved in an email storage format used for organizing email messages in a single document. It saves messages in a concatenated format where each message is stored after another, starting with the “From” header. mbox files were originally used by Unix hosts but are now supported by other email applications, including Apple Mail and Mozilla Thunderbird.
Version 1.x of Apple Mail used MBOX-files and stored them in the /User/Library/Mail/Mailboxes/ directory. However, Mail 2.0 and later uses the emlx format, which separates the messages into individual files in order they could be searched by Spotlight.