mbox(5)               (February 19th, 2002)               mbox(5)

          mbox - Format for mail message storage.

          This document describes the format traditionally used by
          Unix hosts to store mail messages locally.  mbox files typi-
          cally reside in the system's mail spool, under various names
          in users' Mail directories, and under the name mbox in
          users' home directories.

          An mbox is a text file containing an arbitrary number of e-
          mail messages.  Each message consists of a postmark, fol-
          lowed by an e-mail message formatted according to RFC822,
          RFC2822. The file format is line-oriented. Lines are sepa-
          rated by line feed characters (ASCII 10).

          A postmark line consists of the four characters "From", fol-
          lowed by a space character, followed by the message's enve-
          lope sender address, followed by whitespace, and followed by
          a time stamp. This line is often called From_ line.

          The sender address is expected to be addr-spec as defined in
          RFC2822 3.4.1. The date is expected to be date-time as out-
          put by asctime(3).  For compatibility reasons with legacy
          software, two-digit years greater than or equal to 70 should
          be interpreted as the years 1970+, while two-digit years
          less than 70 should be interpreted as the years 2000-2069.
          Software reading files in this format should also be pre-
          pared to accept non-numeric timezone information such as
          "CET DST" for Central European Time, daylight saving time.


           >From example@example.com Fri Jun 23 02:56:55 2000

          In order to avoid misinterpretation of lines in message bod-
          ies which begin with the four characters "From", followed by
          a space character, the mail delivery agent must quote any
          occurrence of "From " at the start of a body line.

          There are two different quoting schemes, the first (MBOXO)
          only quotes plain "From " lines in the body by prepending a
          '>' to the line; the second (MBOXRD) also quotes already
          quoted "From " lines by prepending a '>' (i.e. ">From ",
          ">>From ", ...). The later has the advantage that lines like

           >From the command line you can use the '-p' option

          aren't dequoted wrongly as a MBOXRD-MDA would turn the line

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           >>From the command line you can use the '-p' option

          before storing it. Besides MBOXO and MBOXRD there is also
          MBOXCL which is MBOXO with a "Content-Length:"-field with
          the number of bytes in the message body; some MUAs (like
          neomutt(1)) do automatically transform MBOXO mailboxes into
          MBOXCL ones when ever they write them back as MBOXCL can be
          read by any MBOXO-MUA without any problems.

          If the modification-time (usually determined via stat(2)) of
          a nonempty mbox file is greater than the access-time the
          file has new mail. Many MUAs place a Status: header in each
          message to indicate which messages have already been read.

          Since mbox files are frequently accessed by multiple pro-
          grams in parallel, mbox files should generally not be
          accessed without locking.

          Three different locking mechanisms (and combinations
          thereof) are in general use:

          +o    fcntl(2) locking is mostly used on recent, POSIX-
               compliant systems. Use of this locking method is, in
               particular, advisable if mbox files are accessed
               through the Network File System (NFS), since it seems
               the only way to reliably invalidate NFS clients'

          +o    flock(2) locking is mostly used on BSD-based systems.

          If multiple methods are combined, implementors should make
          sure to use the non-blocking variants of the fcntl(2) and
          flock(2) system calls in order to avoid deadlocks.

          If multiple methods are combined, an mbox file must not be
          considered to have been successfully locked before all indi-
          vidual locks were obtained. When one of the individual lock-
          ing methods fails, an application should release all locks
          it acquired successfully, and restart the entire locking
          procedure from the beginning, after a suitable delay.

          The locking mechanism used on a particular system is a mat-
          ter of local policy, and should be consistently used by all
          applications installed on the system which access mbox
          files. Failure to do so may result in loss of e-mail data,
          and in corrupted mbox files.

               $LOGNAME's incoming mail folder.

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               user's archived mail messages, in his $HOME directory.

               A directory in user's $HOME directory which is commonly
               used to hold mbox format folders.

          neomutt(1), fcntl(2), flock(2), link(2), stat(2),
          asctime(3), maildir(5), mmdf(5), RFC822, RFC976, RFC2822

          Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen

          The mbox format occurred in Version 6 AT&T Unix.
          A variant of this format was documented in RFC976.

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