MBOX (sometimes known as Berkeley format) is a generic term for a family of related file formats used for storing collections of electronic mail messages. MBOX formats store all of the messages of an entire folder (not an entire mailbox) in a single database file and new messages are appended to the end of the file. Each message is immediately prefaced by a separation line and terminated by an empty line. Only the first message in an MBOX database file will be prefaced by a separator line, while every other message will begin with two end-of-line sequences (one at the end of the message itself, and another to mark the end of the message within the MBOX database file stream) and a separator line (marking the new message). The end of the database file is implicitly reached when no more message data or separator lines are found.
A message encoded in MBOX format begins with a "From " line, continues with a series of non-"From " lines, and ends with a blank line. A "From " line means any line in the message or header that begins with the five characters 'F', 'r', 'o', 'm', and ' ' (space). The "From " line structure is From sender date moreinfo:
- sender, usually the envelope sender of the message (e.g., [email protected]) is one word without spaces or tabs
- date is the delivery date of the message which always contains exactly 24 characters in Standard C asctime format (i.e. in English, with the redundant weekday, and without timezone information)
- moreinfo is optional and it may contain arbitrary information.
After the "From " line is the message itself in RFC 5322 format. The final line is a completely blank line with no spaces or tabs.
There are four variants of MBOX: MBOXO, MBOXRD, MBOXCL and MBOXCL2. The four versions all build on the common MBOX structure and are differentiated primarily by changes to the "From " line and and the use of the "Content Length:" field in the message header in determining the start of a new message within the aggregated file. Moreover, the versions and tool sets for one version are not necessary compatible with one another. See General section for incompatibility details.
MBOX files also include the message attachments, if any, in their original MIME format.