mbox is a generic term for a family of relate file formats used for holding collections of electronic mail messages. All messages in an mbox mailbox are concatenated and stored as plain text in a single file. The beginning of each message is indicated by a line whose first five characters consist of "From" followed by a space (the so-called "From_ line" or "'From ' line") and the return path e-mail address. A blank line is appended to the end of each message. For a while, the mbox format was popular because text processing tools can be readily used on the plain text files used to store the e-mail messages.