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       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
       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.


        >From 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
       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.


              $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), asctime(3), maildir(5),
       mmdf(5), RFC822, RFC976, RFC2822


       Thomas    Roessler    <>,    Urs     Janssen


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