.MBOX File Extension

Email storage format used for organizing email messages in a single text file; saves messages in a concatenated format in which each message is stored after another, starting with the "From" header; originally used by Unix hosts but now supported by other email applications, including Apple Mail.

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 so that they can be searched by Spotlight.