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mbox is a generic term for a family of related file formats used for holding collections of email messages, first implemented for Fifth Edition Unix.

All messages in an mbox mailbox are concatenated and stored as plain text in a single file. Each message starts with the four characters "From" followed by a space (the so named "From_ line") and the sender's email address. RFC 4155 defines that a UTC timestamp follows after another separating space character.

Unlike the Internet protocols used for the exchange of email, the format used for the storage of email has never been formally defined through the RFC standardization mechanism and has been entirely left to the developer of an email client. However, the POSIX standard defined a loose frame in conjunction with the mailx program. In 2005 finally, the application/mbox media type was standardized as RFC 4155, and hints that mbox stores mailbox messages in their original Internet Message (RFC 2822) format, except for the used newline character, seven-bit clean data storage, and the requirement that each newly added message is terminated with a completely empty line within the mbox database.

A format similar to mbox is the MH Message Handling System. Other systems, such as Microsoft Exchange Server and the Cyrus IMAP server store mailboxes in centralised databases managed by the mail system and not directly accessible by individual users. The maildir mailbox format is often cited as an alternative to the mbox format for network email storage systems.