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 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.

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:

       •      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.

       •      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 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

       neomutt(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.