NAME

       mbox - file containing mail messages

INTRODUCTION

       The  most  common format for storage of mail messages is mbox format.  An mbox is a single
       file containing zero or more mail messages.

MESSAGE FORMAT

       A message encoded in mbox format begins with a From_ line, continues with a series of non-
       From_ lines, and ends with a blank line.  A From_ line means any line that begins with the
       characters F, r, o, m, space:

            From god@heaven.af.mil Sat Jan  3 01:05:34 1996
            Return-Path: <god@heaven.af.mil>
            Delivered-To: djb@silverton.berkeley.edu
            Date: 3 Jan 1996 01:05:34 -0000
            From: God <god@heaven.af.mil>
            To: djb@silverton.berkeley.edu (D. J. Bernstein)

            How's that mail system project coming along?

       The final line is a completely blank line (no spaces or tabs).  Notice  that  blank  lines
       may also appear elsewhere in the message.

       The  From_  line  always  looks like From envsender date moreinfo.  envsender is one word,
       without spaces or tabs; it is usually the envelope sender of the  message.   date  is  the
       delivery date of the message.  It always contains exactly 24 characters in asctime format.
       moreinfo is optional; it may contain arbitrary information.

       Between the From_ line and the blank line is a message in RFC 822 format, as described  in
       qmail-header(5), subject to >From quoting as described below.

HOW A MESSAGE IS DELIVERED

       Here is how a program appends a message to an mbox file.

       It  first  creates  a From_ line given the message's envelope sender and the current date.
       If the envelope sender is empty (i.e., if this is a  bounce  message),  the  program  uses
       MAILER-DAEMON  instead.   If  the  envelope sender contains spaces, tabs, or newlines, the
       program replaces them with hyphens.

       The program then copies the message, applying >From quoting to each line.   >From  quoting
       ensures  that  the  resulting  lines  are not From_ lines: the program prepends a > to any
       From_ line, >From_ line, >>From_ line, >>>From_ line, etc.

       Finally the program appends a blank line to the message.  If the last line of the  message
       was a partial line, it writes two newlines; otherwise it writes one.

HOW A MESSAGE IS READ

       A  reader  scans  through  an mbox file looking for From_ lines.  Any From_ line marks the
       beginning of a message.  The reader should not attempt to take advantage of the fact  that
       every From_ line (past the beginning of the file) is preceded by a blank line.

       Once  the  reader  finds a message, it extracts a (possibly corrupted) envelope sender and
       delivery date out of the From_ line.  It then reads until the next From_ line  or  end  of
       file,  whichever  comes first.  It strips off the final blank line and deletes the quoting
       of >From_ lines and >>From_ lines and so on.  The result is an RFC 822 message.

COMMON MBOX VARIANTS

       There are many variants of mbox format.  The variant described  above  is  mboxrd  format,
       popularized by Rahul Dhesi in June 1995.

       The  original  mboxo  format quotes only From_ lines, not >From_ lines.  As a result it is
       impossible to tell whether

            From: djb@silverton.berkeley.edu (D. J. Bernstein)
            To: god@heaven.af.mil

            >From now through August I'll be doing beta testing.
            Thanks for your interest.

       was quoted in the original message.  An mboxrd reader will always strip off the quoting.

       mboxcl format is like mboxo format, but includes a Content-Length field with the number of
       bytes  in  the  message.   mboxcl2  format is like mboxcl but has no >From quoting.  These
       formats are used by SVR4 mailers.  mboxcl2 cannot be read safely by mboxrd readers.

UNSPECIFIED DETAILS

       There are many locking mechanisms for  mbox  files.   qmail-local  always  uses  flock  on
       systems that have it, otherwise lockf.

       The  delivery  date  in  a  From_  line  does not specify a time zone.  qmail-local always
       creates the delivery date in GMT so that mbox files can be  safely  transported  from  one
       time zone to another.

       If the mtime on a nonempty mbox file is greater than the atime, the file has new mail.  If
       the mtime is smaller than the atime, the new mail has been read.  If the atime equals  the
       mtime, there is no way to tell whether the file has new mail, since qmail-local takes much
       less than a second to run.  One solution is for a mail  reader  to  artificially  set  the
       atime  to  the  mtime plus 1.  Then the file has new mail if and only if the atime is less
       than or equal to the mtime.

       Some mail readers place Status fields in each message to indicate which messages have been
       read.

SEE ALSO

       maildir(5), qmail-header(5), qmail-local(8)

                                                                                          mbox(5)