Mbox is a generic term for a family of related used for holding collections of email messages. It was first implemented in Research Unix.
All messages in an mbox mailbox are Concatenation 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.
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 centralized 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 networked email storage systems.
Mail storage protocols
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 framework in conjunction with the
mailx 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.
File locking
Because more than one messages are stored in a single file, some form of file 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, including fcntl() and lockf(). This does not work well with network mounted file systems, such as the Network 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.
As a patch format
In open source development, it is common to send patches in the diff format to a mailing list for discussion. The diff format allows for irrelevant "headers", such as mbox data, to be added.
See also
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Maildir
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MIX (email)
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MH Message Handling System
Further reading
