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mbox(5)                          User Manuals                          mbox(5)

NAME
       mbox - Format for mail message storage.

DESCRIPTION
       This document describes the format traditionally used by Unix hosts to
       store mail messages locally.  mbox files typically 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, followed by an e-mail
       message formatted according to RFC822, RFC2822. The file format is
       line-oriented. Lines are separated by line feed characters (ASCII 10).

       A postmark line consists of the four characters "From", followed by a
       space character, followed by the message's envelope 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 output 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
       prepared to accept non-numeric timezone information such as "CET DST"
       for Central European Time, daylight saving time.

       Example:

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

       In order to avoid misinterpretation of lines in message bodies 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 into

        >>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 mutt(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.

LOCKING
       Since mbox files are frequently accessed by multiple programs 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' caches.

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

       o      Dotlocking is used on all kinds of systems. In order to lock an
              mbox file named folder, an application first creates a temporary
              file with a unique name in the directory in which the folder
              resides. The application then tries to use the link(2) system
              call to create a hard link named folder.lock to the temporary
              file. The success of the link(2) system call should be
              additionally verified using stat(2) calls. If the link has
              succeeded, the mail folder is considered dotlocked. The
              temporary file can then safely be unlinked.

              In order to release the lock, an application just unlinks the
              folder.lock file.

       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 individual locks were
       obtained. When one of the individual locking 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 matter 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.

FILES
       /var/spool/mail/$LOGNAME
              $LOGNAME's incoming mail folder.

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

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

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

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

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

Unix                          February 19th, 2002                      mbox(5)