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Mbox is a generic term for a family of relatedfile formats used for holding collections ofemail messages. It was first implemented inFifth Edition Unix.
All messages in an mbox mailbox areconcatenated and stored asplain 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 aUTC timestamp follows after another separating space character.[1]
A format similar to mbox is theMH Message Handling System. Other systems, such asMicrosoft Exchange Server and theCyrus IMAP server, store mailboxes in centralized databases managed by the mail system and not directly accessible by individual users. Themaildir mailbox format is often cited as an alternative to thembox format for networked email storage systems.
Unlike the Internet protocols used for the exchange of email, the format used for the storage of email has never been formally defined through theRFC standardization mechanism and has been entirely left to the developer of an email client. However, thePOSIX standard defined a loose framework in conjunction with themailx program. In 2005, the application/mbox media type was standardized as RFC 4155, which hinted 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.[1][2]
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Because more than one messages are stored in a single file, some form offile locking is needed to avoid the corruption that can result from two or more processes modifying the mailbox simultaneously. This could happen if a network email delivery program delivers a new message at the same time as a mail reader is deleting an existing message.
Various mutually incompatible mechanisms have been used by different mbox formats to enable message file locking, includingfcntl() andlockf(). This does not work well with network mounted file systems, such as theNetwork File System (NFS), which is why traditionally Unix used additional "dot lock" files, which could be created atomically even over NFS.
Mbox files should also be locked while they are being read. Otherwise, the reader may see corrupted message contents if another process is modifying the mbox at the same time, even though no actual file corruption occurs.
Inopen source development, it is common to sendpatches in thediff format to amailing list for discussion. The diff format allows for irrelevant "headers", such as mbox data, to be added.[3][4]Version control systems likegit have support for generating mbox-formatted patches and for sending them to the list as emails in a thread.[5][6]
