Mbox is a generic term for a family of related file formats used for holding collections of email messages. It was first implemented in 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-called "From_ line") and the sender's email address. RFC 4155 defines that a UTC timestamp follows after another separating space character.