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What is an MBOX file?
An MBOX file is an email mailbox, saved in a mail storage format that lists an entire inbox's messages in a single text file. It saves messages in a concatenated format where each message is listed after another, starting with the From header. MBOX files were originally used only by Unix hosts, but other email applications, including Apple Mail and Mozilla Thunderbird, now also support them.
Version 1.x of Apple Mail used MBOX files and stored them in the /User/Library/Mail/Mailboxes/ directory. Mail 2.0 and later versions use the .EMLX format, which separates messages into individual files so they can be searched by Spotlight.
However, Apple Mail still exports mailboxes as MBOX files. If you right-click a mailbox and select Export Mailbox..., Apple Mail will save that mailbox's contents as an MBOX file.
NOTE: Opera Mail, which was discontinued in 2016, also stored messages in the MBOX format.
Common MBOX Filenames
INBOX.mbox - The name given to the primary Inbox mail folder in Apple Mail, macOS's default email program.
How to open an MBOX file
How to import MBOX files into Apple Mail and Mozilla Thunderbird
To import an MBOX file into Apple Mail, select File → Import Mailboxes and then follow the prompts to import your MBOX file. The messages your MBOX file contains will appear in the Import mailbox.
To import an MBOX file into Mozilla Thunderbird, you must first install the ImportExportTools NG add-on. Then, open Thunderbird and select Tools → ImportExportTools → Import MBOX File and follow the prompts to import your MBOX file. The messages your MBOX file contains will appear in the Local Folders inbox.