from Wikipedia, the free encyclopedia

mbox , from English "mailbox" ("Postfach"), is a file format with which several e-mails are saved together in one file . The format is mostly known from Unix.


The e-mails are stored in the file in the order in which they are received, each e-mail starting with a separator line and typically followed by a blank line. The separator line usually begins with the sequence of symbols “From”, followed by a space, an e-mail address, another space and the time stamp of the receipt.

Many implementations deviate from this standard. Four variants developed under Unix that are not compatible are documented.

The mbox files usually do not have an extension , but occasionally .mbox or .mbx are used.


  • Fewer larger files require less storage space than many smaller files because fewer data blocks are incompletely occupied.
  • Programs can read a single larger file faster than many smaller files because there is no need to open and close multiple files.


  • Programs cannot track any changes without completely reading all of the e-mails contained in the mbox file.
  • Simultaneous changes, for example deleting an e-mail while a new one arrives, must be prevented by appropriate locks in order not to destroy the file.
  • In connection with such data backups , which do not copy changed data blocks but traditionally changed files, a relatively large amount of storage space is required, even if only individual e-mails have been deleted.
  • If a virus scanner detects (correctly or incorrectly ) an email stored in the mbox file as malware , this can lead to the mbox file being blocked or, in the worst case, to a defective mbox after an unsuitable repair of the mbox file by the virus scanner -File so that with the help of the virus scanner, a single, virus-contaminated email can make all other emails stored in the same mbox file illegible.

The newer concept Maildir avoids these disadvantages by saving each email individually in a separate file.


This traditional method of storing emails dates back to the early 1960s and is widely used. An official definition of this data format, RFC 4155 , has existed since 2005 .

Individual evidence

  1. a b c d e f g E. Hall: RFC 4155 - The application / mbox Media Type . Internet Society. September 2005. Retrieved June 4, 2009.
  2. mbox - file containing mail messages . Russell Nelson. Retrieved June 4, 2009.
  3. Jonathan de Boyne Pollard: "mbox" is a family of several mutually incompatible mailbox formats. . In: Frequently Given Answers . 2004. Retrieved June 4, 2009.
  4. ^ Ray Tomlinson: The First Email . Retrieved June 4, 2009.

Web links