What is a MBOX File Type?
Apple Mail's version 1.x used MBOX files and stored them in the directory / User / Library / Mailbox / Mailbox. Mail 2.0 and later versions, however, use the .EMLX format, which separates messages into individual files so that they can be searched by Spotlight.
Opera Mail also stored messages in MBOX format, which was shut down in 2016.
What is a Default MBOX File Name?
INBOX.mbox - The name given to the first Inbox mail folder in Apple Mail, the default mail program for Mac OS X.