mbox(5)                          User Manuals                          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 typically reside in  the  sys-
       tem'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  mes-
       sages.  Each message consists of a postmark, followed by an e-mail mes-
       sage formatted according to RFC5322. 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 of-
       ten called From_ line.

       The sender address is expected to be addr-spec as  defined  in  RFC5322
       3.4.1.  The date is expected to be date-time as defined in RFC5322 3.3.
       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 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.

       Since mbox files are frequently accessed by multiple programs in paral-
       lel, 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 sys-
              tems. 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.

       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 re-
              sides. 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 ob-
       tained. When one of the individual locking methods fails,  an  applica-
       tion 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.

              $LOGNAME's incoming mail folder.

              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.

       mutt(1), fcntl(2), flock(2),  link(2),  stat(2),  maildir(5),  mmdf(5),
       RFC976, RFC4155, RFC5322

       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 and RFC4155.

Unix                          November 4th, 2013                       mbox(5)

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