MBOX

MBOX is a generic term for a family of plain-text file formats: it stores a collection of email messages in a single concatenated file, originating from Fifth Edition Unix.

MBOX is the historical standard for email storage, a family of related plain-text formats (e.g., MBOXO, MBOXRD, MBOXCL) that concatenate all messages from an email folder into one file. The format delineates each message with a distinct 'From ' line, which includes the sender and a 24-character asctime-format timestamp. Officially standardized as the `application/mbox` media type in RFC 4155 (September 2005), this simple, universal structure makes it critical for cross-platform email migration, archival, and eDiscovery, with major clients like Apple Mail and Mozilla Thunderbird relying on it for local storage.

https://www.rfc-editor.org/rfc/rfc4155.html