There is only one file type that uses the .MBOX extension:
Email Mailbox File
An MBOX file is an email mailbox saved in a mail storage format used for organizing email messages in a single text file. 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 so that they can be searched by Spotlight.
Opera Mail, which was discontinued in 2016, also stored messages in the MBOX format.
INBOX.mbox - The name given to the primary Inbox mail folder in Apple Mail, the default email program for Mac OS X.
Updated: April 28, 2019